Implement WASI "preview0" in wasmtime-wasi

This commit implements the `wasi_unstable` module, sometimes referred to
as "preview0", in the `wasmtime-wasi` crate. Previously this was only
implemented by the `wasi-common` crate but now this is implemented for
both meaning that the switch to preview2 won't lose this functionality.

The preview0 WITX files are vendored like the preview1 files and the
implementation of preview0 is exclusively implemented by delegating to
the preview1 implementation.

crates/cli-flags/src/lib.rs

@@ -262,7 +262,8 @@ wasmtime_option_group! {
         pub inherit_network: Option<bool>,
         /// Indicates whether `wasi:sockets/ip-name-lookup` is enabled or not.
         pub allow_ip_name_lookup: Option<bool>,
-
+        /// Allows imports from the `wasi_unstable` core wasm module.
+        pub preview0: Option<bool>,
     }
 
     enum Wasi {

crates/wasi/src/preview2/mod.rs

@@ -30,6 +30,8 @@ mod network;
 pub mod pipe;
 mod poll;
 #[cfg(feature = "preview1-on-preview2")]
+pub mod preview0;
+#[cfg(feature = "preview1-on-preview2")]
 pub mod preview1;
 mod random;
 mod stdio;