Create a C API posix-compatible malloc/free wrapper crate

[fitzgen]

Summary

Add a C API crate that wraps wee_alloc and provides posix malloc and free.

Motivation

C and C++ projects targeting wasm could benefit from wee_alloc too, and more users = more bug reports and reliability work and all that good stuff.

Details

• New crate at the top level of the repo, depending on wee_alloc

• Wraps a wee_alloc global allocator and exposes malloc and free (and I think realloc is part of the posix spec too?)

[pepyakin]

and I think realloc is part of the posix spec too?

I believe that not only realloc but also calloc.

There is some API mismatch, Rust allocators takes Layout argument for alloc and dealloc, so, I guess on call to free before calling to dealloc we need to somehow obtain Layout for this particular allocation?

[fitzgen]

Yeah.

An easy, expedient solution would be to have the posix crate allocate n + 1 words and store the size just before the real allocation.

An alternative, probably-better solution would be to tighten up the size classes' free list invariant to say that allocations from these lists must be exactly the size class's size. Right now, because we don't ensure that the blocks that populate a size class's free list are a multiple of the size class, and we never split a cell smaller than the size class's size, so the last cell from a block can have some remainder bloat size within it. If we fixed this and tightened the invariant, then we could just look at the cell's header's size to determine which free list to put it back in, and would not need to store any extra data.