compiler: add support for recursive function types

[aykevl]

This adds support for a construct like this:

type foo func(fn foo)

Unfortunately, LLVM cannot create function pointers that look like this.
LLVM only supports named types for structs (not for pointers) and thus
can't add a pointer to a function type of the same type to a parameter
of that function type.

The fix is simple: cast all function pointers to a void function, in
LLVM IR:

void ()*

Raw function pointers are cast to this type before storing, and cast
back to the regular function type before calling. This means that
function parameters will never refer to its own type because raw
function types are fixed at that one type.

Somehow, this does have an effect on binary size in some cases. The
effect is small and goes both ways. On top of that, there is work
underway in LLVM which would make all pointer types opaque (without a
pointee type). This would make this whole commit useless and therefore
should fix any size increases that might happen.
https://llvm.org/docs/OpaquePointers.html

[aykevl]

Rebased to fix the merge conflict.
This PR solves #2172.

[niaow]

I just gave this a rebase so that #2209 is applied and CI should run (I hope this was okay?)

[niaow]

This seems to be working as expected. LGTM.

[deadprogram]

Thanks @aykevl for working on this and thanks @niaow for review.

Now merging.