Update binary encoding to use varuint32 for flags (#35)

This commit fixes #34 by specifying that the flags field (which
indicates if a segment is passive) is a `varuint32` instead of a
`uint8`. It was discovered in #34 that the memory index located at that
position today is a `varuint32`, which can be validly encoded as `0x80
0x00` in addition to `0x00` (in addition to a number of other
encodings). This means that if the first field were repurposed as a
single byte of flags, it would break these existing modules that work
today.

It's not currently known how many modules in the wild actually take
advantage of such an encoding, but it's probably better to be safe than
sorry!

Closes #34

Author: alexcrichton

proposals/bulk-memory-operations/Overview.md

@@ -168,13 +168,12 @@ currently has a collection of segments, each of which has a memory index, an
 initializer expression for its offset, and its raw data.
 
 Since WebAssembly currently does not allow for multiple memories, the memory
-index of each segment must be zero, which when represented as a `varuint32` is
-a single zero byte. We can repurpose this byte as a flags field.
+index of each segment must be zero. We can repurpose this 32-bit integer as a
+flags field where new meaning is attached to nonzero values.
 
-When the least-significant bit of this new flags field is `1`, this segment is
-_passive_. A passive segment will not be automatically copied into the
-memory or table on instantiation, and must instead be applied manually using
-the following new instructions:
+When the new flags field is `1`, this segment is _passive_. A passive segment
+will not be automatically copied into the memory or table on instantiation, and
+must instead be applied manually using the following new instructions:
 
 * `memory.init`: copy a region from a data segment
 * `table.init`: copy a region from an element segment
@@ -195,7 +194,7 @@ The new encoding of a data segment is now:
 
 | Field | Type | Description |
 |------|-------|-------------|
-| flags | `uint8` | Flags for passive and presence of fields below, only values of 0, 1, and 2 are valid |
+| flags | `varuint32` | Flags for passive and presence of fields below, only values of 0, 1, and 2 are valid |
 | index | `varuint32`? | Memory index this segment is for, only present if `flags` is 2, otherwise the index is implicitly 0 |
 | offset | `init_expr`? | an `i32` initializer expression for offset, not present if `flags & 0x1` is set |
 | size | `varuint32` | size of `data` (in bytes) |