Propagate module-linking types to wasmtime (#2115)

This commit adds lots of plumbing to get the type section from the
module linking proposal plumbed all the way through to the `wasmtime`
crate and the `wasmtime-c-api` crate. This isn't all that useful right
now because Wasmtime doesn't support imported/exported
modules/instances, but this is all necessary groundwork to getting that
exported at some point. I've added some light tests but I suspect the
bulk of the testing will come in a future commit.

One major change in this commit is that `SignatureIndex` no longer
follows type type index space in a wasm module. Instead a new
`TypeIndex` type is used to track that. Function signatures, still
indexed by `SignatureIndex`, are then packed together tightly.

tests/all/module_linking.rs

@@ -44,3 +44,11 @@ fn compile() -> Result<()> {
     assert_eq!(m.exports().len(), 0);
     Ok(())
 }
+
+#[test]
+fn types() -> Result<()> {
+    let engine = engine();
+    Module::new(&engine, "(module (type (module)))")?;
+    Module::new(&engine, "(module (type (instance)))")?;
+    Ok(())
+}