Add serdes API

[cjihrig]

It would be useful if embedders didn't need to worry about serializing and deserializing between WASI data structures and the WASM memory. For example, Node.js currently does this:

uvwasi_fdstat_t stats;
uvwasi_errno_t err = uvwasi_fd_fdstat_get(&wasi->uvw_, fd, &stats);

if (err == UVWASI_ESUCCESS) {
  wasi->writeUInt8(memory, stats.fs_filetype, buf);
  wasi->writeUInt16(memory, stats.fs_flags, buf + 2);
  wasi->writeUInt64(memory, stats.fs_rights_base, buf + 8);
  wasi->writeUInt64(memory, stats.fs_rights_inheriting, buf + 16);
}

The wasi->write*() calls could be replaced with a uvwasi_serdes_write_fdstat_t(memory, stats, buf).

Another option could be to provide wrappers for each function (uvwasi_serdes_fd_fdstat_get(), etc.). This would take care of bounds checking, reading from memory, executing the WASI API calls, writing back to memory, and forwarding the correct return value. This would be the simplest thing for embedders, but would also require a wrapper function for every WASI API call.

Potentially useful reference: https://github.com/WebAssembly/tool-conventions/blob/master/BasicCABI.md

Any thoughts or opinions from others?

[tniessen]

I think the first option sounds better to me, simply because we won't need an additional layer for every single call. Having uvwasi_serdes_write_* seems like a good idea!

[cjihrig]

#94 has landed, but I'm leaving this open, as there are still a few things I'd like to address in the new serdes API:

1. It looks like add serdes API #94 didn't include support for uvwasi_ciovec_t and uvwasi_iovec_t.
2. It would be nice to support serializing/deserializing arrays, which ends up overlapping a lot with item 1.