Document WASM usage

[indo-dev-0]

It would be nice if there was documentation of the WASM/WASI support. Even just adding some WASI tests would be helpful.

[daanx]

Ah, there is WASM support but it is not regularly tested. However, it should just work. The only problem at this point is if you do large allocations > 64MiB the memory is not returned because currently in WASM you can only grow memory. The best remedy at this point is to pre-reserve a large "mimalloc arena" from which mimalloc can allocate any block size (huge or small). For example, at the start of the program, you could use:

mi_reserve_os_memory( 1UL << 30 /*1GiB*/,  false /*commit*/, true /*allow large*/ );

which pre-reserves 1GiB of memory in one go from the WASM heap which can then be managed well by mimalloc for large allocations as well. (also, this works best with the dev-slice branch)