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-c/src/lib.rs

@@ -0,0 +1,18 @@
+extern crate cast;
+extern crate cranelift_codegen;
+extern crate cranelift_entity;
+extern crate cranelift_wasm;
+extern crate target_lexicon;
+extern crate wasmtime_environ;
+extern crate wasmtime_jit;
+extern crate wasmtime_runtime;
+#[macro_use]
+extern crate log;
+
+mod host;
+mod instantiate;
+mod syscalls;
+mod translate;
+mod wasm32;
+
+pub use instantiate::instantiate_wasi_c;