wiggle: emit a metadata module containing witx document (#1387)

* wiggle: emit a metadata module containing witx document

* wiggle: put metadata module behind a wiggle_metadata feature

* wasi-common: add wiggle_metadata feature and optional witx dep

* refactor according to alex's advice

* wasi-common: make snapshots pub

* wasi-common: i do need a wiggle_metadata feature to be available

* Tweak features and such

* wiggle: fix tests by passing metadata flag to wiggle-runtime

* wiggle: need to move wiggle-runtime to a non-dev dependency

so that the feature resolves for external users of the crates

Co-authored-by: Alex Crichton <alex@alexcrichton.com>

crates/wiggle/src/lib.rs

@@ -87,6 +87,12 @@ pub fn from_witx(args: TokenStream) -> TokenStream {
     config.witx.make_paths_relative_to(
         std::env::var("CARGO_MANIFEST_DIR").expect("CARGO_MANIFEST_DIR env var"),
     );
+
+    #[cfg(feature = "wiggle_metadata")]
+    {
+        config.emit_metadata = true;
+    }
+
     let doc = witx::load(&config.witx.paths).expect("loading witx");
     TokenStream::from(wiggle_generate::generate(&doc, &config))
 }