Implement WASI C API.

This commit implements an initial WASI C API that can be used to instantiate
and configure a WASI instance from C.

This also implements a `WasiBuilder` for the C# API enabling .NET hosts to bind
to Wasmtime's WASI implementation.

crates/c-api/Cargo.toml

@@ -18,3 +18,5 @@ doctest = false
 
 [dependencies]
 wasmtime = { path = "../api" }
+wasi-common = { path = "../wasi-common" }
+wasmtime-wasi = { path = "../wasi" }