Implement a new use syntax

This commit implements a new `use` syntax which changes

  (use "foo.witx")

to

  (use $some $type $names from $foo)

This change ended up being much larger than originally intended. While
at a surface level switching this style of `use` statements isn't too
bad it has large ramifications on the mental model of how to interpret
and work with `*.witx` files. This necessitated some internal
refactorings which ended up as a bit of a yak shave. The other changes
here are:

* The `polyfill` module is removed. I confirmed with Pat that this isn't
  used anymore and this came about trying to update it from the below
  refactorings.
* The `render` module is removed. Documents can no longer be serialized
  to a standalone s-expression because they can refer to other documents
  in a structured manner. It's also believed there are no current users
  of this functionality.
* The `representation` module was removed since it was largely only used
  by the `polyfill` module.
* The `Document` type was renamed to `Module`, and the ability for a
  document to contain multiple modules has been removed. When working
  with multiple modules this must now be done so explicitly instead of
  having it all lumped into one parsed object. This means that
  `Module` is now a list of types and a list of functions.
* Documentation has changed to document a set of modules instead of a
  single module. Additionally this is nontrivially changed to load types
  transitively from all modules referenced to ensure that all relevant
  types are documented.
* Internal refactorings in the validation phase have been done to
  streamline a few things and consolidate where possible with this new
  structuring.
* `InterfaceFunc` is now named `Function`
* `InterfaceFuncParam` is now named `Param`
* `Module` has a `name` (as before) which is now inferred from the
  filename it's loaded from.
* `Module` has a `ModuleId` now to represent types that are defined in
  other modules, where `ModuleId` is intended to be a unique identifier
  amongst a set of modules. For now it's just a path name but can likely
  get more fancy in the future if needed.

All `*.witx` files have been updated to the new syntax in this
repository. Lots of changes look like they happened to the proposal
documentation, but that's just because the documentation order of types
has been shuffled around. I've checked to make sure no actual items were
lost from the documentation.

Closes #378
Closes #379

Author: alexcrichton

phases/ephemeral/docs.md

@@ -3,12 +3,9 @@
 ;; This is a `witx` file. See [here](https://github.com/WebAssembly/WASI/tree/master/docs/witx.md)
 ;; for an explanation of what that means.
 
-(use "typenames.witx")
+(use $errno $size from $typenames)
 
 (module $wasi_ephemeral_args
-  ;;; Linear memory to be accessed by WASI functions that need it.
-  (import "memory" (memory))
-
   ;;; Read command-line argument data.
   ;;; The size of the array should match that returned by `sizes_get`.
   ;;; Each argument is expected to be `\0` terminated.

phases/ephemeral/witx/wasi_ephemeral_args.witx

@@ -5,12 +5,9 @@
 ;; This is a `witx` file. See [here](https://github.com/WebAssembly/WASI/tree/master/docs/witx.md)
 ;; for an explanation of what that means.
 
-(use "typenames.witx")
+(use $clockid $timestamp $errno from $typenames)
 
 (module $wasi_ephemeral_clock
-  ;;; Linear memory to be accessed by WASI functions that need it.
-  (import "memory" (memory))
-
   ;;; Return the resolution of a clock.
   ;;; Implementations are required to provide a non-zero value for supported clocks. For unsupported clocks,
   ;;; return `errno::inval`.

phases/ephemeral/witx/wasi_ephemeral_clock.witx

@@ -3,12 +3,9 @@
 ;; This is a `witx` file. See [here](https://github.com/WebAssembly/WASI/tree/master/docs/witx.md)
 ;; for an explanation of what that means.
 
-(use "typenames.witx")
+(use $errno $size from $typenames)
 
 (module $wasi_ephemeral_environ
-  ;;; Linear memory to be accessed by WASI functions that need it.
-  (import "memory" (memory))
-
   ;;; Read environment variable data.
   ;;; The sizes of the buffers should match that returned by `sizes_get`.
   ;;; Key/value pairs are expected to be joined with `=`s, and terminated with `\0`s.

phases/ephemeral/witx/wasi_ephemeral_environ.witx

@@ -5,12 +5,9 @@
 ;; This is a `witx` file. See [here](https://github.com/WebAssembly/WASI/tree/master/docs/witx.md)
 ;; for an explanation of what that means.
 
-(use "typenames.witx")
+(use * from $typenames)
 
 (module $wasi_ephemeral_fd
-  ;;; Linear memory to be accessed by WASI functions that need it.
-  (import "memory" (memory))
-
   ;;; Provide file advisory information on a file descriptor.
   ;;; Note: This is similar to `posix_fadvise` in POSIX.
   (@interface func (export "advise")

phases/ephemeral/witx/wasi_ephemeral_fd.witx

@@ -5,12 +5,9 @@
 ;; This is a `witx` file. See [here](https://github.com/WebAssembly/WASI/tree/master/docs/witx.md)
 ;; for an explanation of what that means.
 
-(use "typenames.witx")
+(use * from $typenames)
 
 (module $wasi_ephemeral_path
-  ;;; Linear memory to be accessed by WASI functions that need it.
-  (import "memory" (memory))
-
   ;;; Create a directory.
   ;;; Note: This is similar to `mkdirat` in POSIX.
   (@interface func (export "create_directory")

phases/ephemeral/witx/wasi_ephemeral_path.witx

@@ -5,12 +5,9 @@
 ;; This is a `witx` file. See [here](https://github.com/WebAssembly/WASI/tree/master/docs/witx.md)
 ;; for an explanation of what that means.
 
-(use "typenames.witx")
+(use $event $subscription $size $errno from $typenames)
 
 (module $wasi_ephemeral_poll
-  ;;; Linear memory to be accessed by WASI functions that need it.
-  (import "memory" (memory))
-
   ;;; Concurrently poll for the occurrence of a set of events.
   ;;;
   ;;; If `nsubscriptions` is 0, returns `errno::inval`.

phases/ephemeral/witx/wasi_ephemeral_poll.witx

@@ -5,7 +5,7 @@
 ;; This is a `witx` file. See [here](https://github.com/WebAssembly/WASI/tree/master/docs/witx.md)
 ;; for an explanation of what that means.
 
-(use "typenames.witx")
+(use $exitcode from $typenames)
 
 (module $wasi_ephemeral_proc
   ;;; Terminate the process normally. An exit code of `$exitcode::success`

phases/ephemeral/witx/wasi_ephemeral_proc.witx

@@ -5,12 +5,9 @@
 ;; This is a `witx` file. See [here](https://github.com/WebAssembly/WASI/tree/master/docs/witx.md)
 ;; for an explanation of what that means.
 
-(use "typenames.witx")
+(use $errno $size from $typenames)
 
 (module $wasi_ephemeral_random
-  ;;; Linear memory to be accessed by WASI functions that need it.
-  (import "memory" (memory))
-
   ;;; Write high-quality random data into a buffer.
   ;;; This function blocks when the implementation is unable to immediately
   ;;; provide sufficient high-quality random data.

phases/ephemeral/witx/wasi_ephemeral_random.witx

@@ -5,7 +5,7 @@
 ;; This is a `witx` file. See [here](https://github.com/WebAssembly/WASI/tree/master/docs/witx.md)
 ;; for an explanation of what that means.
 
-(use "typenames.witx")
+(use $errno from $typenames)
 
 (module $wasi_ephemeral_sched
   ;;; Temporarily yield execution of the calling thread.

phases/ephemeral/witx/wasi_ephemeral_sched.witx

@@ -5,12 +5,9 @@
 ;; This is a `witx` file. See [here](https://github.com/WebAssembly/WASI/tree/master/docs/witx.md)
 ;; for an explanation of what that means.
 
-(use "typenames.witx")
+(use * from $typenames)
 
 (module $wasi_ephemeral_sock
-  ;;; Linear memory to be accessed by WASI functions that need it.
-  (import "memory" (memory))
-
   ;;; Receive a message from a socket.
   ;;; Note: This is similar to `recv` in POSIX, though it also supports reading
   ;;; the data into multiple buffers in the manner of `readv`.