migrate from size_t to uvwasi_size_t

[cjihrig]

This commit defines a 32-bit fixed width uvwasi_size_t, and migrates from size_t to uvwasi_size_t.

Fixes: #119

[sbc100]

Sweet! Thanks for the quick fix.

I was also wondering if could generate uvwasi/include/wasi_types.h from the witx files, like we do in wasi-libc:
https://github.com/WebAssembly/wasi-libc/blob/master/libc-bottom-half/headers/public/wasi/api.h

That way its alwasy up-to-date and we can include those asserts that the struct layout matches that in the wasm memory too. I'll open another issue that.. and I'm happy out with implementing it if needed.

[cjihrig]

Yes, I'm definitely in favor generating that file - it's just not something I've had the time to get to.