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/host.rs

@@ -0,0 +1,6 @@
+#![allow(non_camel_case_types, dead_code)]
+
+include!(concat!(env!("OUT_DIR"), "/wasmtime_ssp.rs"));
+
+pub type char = ::std::os::raw::c_char;
+pub type void = ::std::os::raw::c_void;