* Use generated type bindings.
Use the witx API descriptions to generate the bulk of the contents of
host.rs, wasi.rs, and wasi32.rs.
This also prunes out many of the miscellaneous libc definitions from
those files which aren't currently in use by wasi-common. If there's
anything removed that's still needed by someone, it's easy to add things
back in.
* Remove unneeded iovec conversion routines.
Now, test binaries are bundled with the repo, and
just like in CraneStation/wasmtime, the test cases
are generated automatically using build.rs. So all
it takes is to drop a new test binary in the
testsuite dir to get the test case for it generated
(with some caveats to do with handling preopens).
