WASI prototype design, implementation, and documentation.

This adds documents describing the WASI Core API, and an implementation in
Wasmtime.

wasmtime-wasi/README.md

@@ -0,0 +1,8 @@
+This is the `wasmtime-wasi` crate, which implements the
+WebAssembly System Interface (WASI) API.
+
+WASI is greatly inspired by and directly derived from [CloudABI].
+It differs in that it has aspirations to expand to a greater
+scope, and to better support the needs of WebAssembly engines.
+
+[CloudABI]: https://cloudabi.org/