Provide the C WASI implementation as an option.

This adds the C WASI implementation as a new crate, wasmtime-wasi-c,
and adds a command-line flag to the wasmtime command-line driver to
select which WASI implementation to use.

wasmtime-wasi/README.md

@@ -1,5 +1,5 @@
 This is the `wasmtime-wasi` crate, which implements the
-WebAssembly System Interface (WASI) API.
+WebAssembly System Interface (WASI) API in Rust.
 
 WASI is greatly inspired by and directly derived from [CloudABI].
 It differs in that it has aspirations to expand to a greater