Use generated type bindings (#152)

* 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.

.gitmodules

@@ -1,3 +1,6 @@
 [submodule "misc_testsuite"]
 	path = misc_testsuite
 	url = https://github.com/cranestation/wasi-misc-tests
+[submodule "WASI"]
+	path = WASI
+	url = https://github.com/WebAssembly/WASI