Provide the C WASI implementation as an option.

[sunfishcode]

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.

This will make it easier to test the C and Rust versions against each other. I'm finding this useful as I'm debugging the test failures in wasi-common. When we're ready to enable wasi-common by default, we can maybe put the C version behind a feature flag so that it doesn't get built by default (and we don't need the C dependencies by default) and keep it around as long as it's useful.

[kubkon]

Overall looks good to me. One thing though. How about we put the C version behind a target dependency, so that we don't have to disable it manually whenever we want to build wasmtime on Windows? I was thinking of something along the lines of:

let wasi = if args.flag_wasi_common {
    instantiate_wasi("", global_exports, &preopen_dirs, &argv, &environ)
} else {
    #[cfg(unix)]
    {
        wasmtime_wasi_c::instantiate_wasi_c("", global_exports, &preopen_dirs, &argv, &environ)
    }
    #[cfg(not(unix))]
    {
        unimplemented!("wasmtime-wasi-c requires a *nix")
    }
}
.expect("instantiating wasi");

and in Cargo.toml:

[target.'cfg(unix)'.dependencies]
wasmtime-wasi-c = { path = "wasmtime-wasi-c" }

[sunfishcode]

Makes sense. I've now made this change.

[kubkon]

Looks great, thanks!