Style guide

[mikevoronov]

It would good to have a explicit style guide for contributors of this repo (and maybe others in Cranelift). Because there are some divergences between different files (like comment style here and here) and sometimes it hard to make code follow the style guide.

[sunfishcode]

wasi-libc includes codes from several external sources, with differing style conventions, so when adding new code, we generally just follow the style of the nearest code.

That said, I agree we could be more consistent in our own code. I've lately been moving wasi-libc's own .c files from /* to // comments (#112, #110, #109); once those land we could do a follow-up to convert the few remaining places.