fixed wasi support (I think)

[sunny-g]

New to wasi, but I think these changes fix the build errors described in #92.

I likely missed/mis-implemented something, and I committed some cargo fmt changes + deny warnings as well, I can cherry-pick them out and make a new PR if necessary. The relevant changes are:

• adding wasi as a dependency
• replacing __wasi_fd_t with wasi::Fd in sys::wasi::{mod,impl}.rs
• updating send and recv to use wasi::sock_send and wasi::sock_recv in ext.rs, lines 1293 and 1346.

Feel free to let me know what to fix.