Move the signature registry into Engine.

This commit moves the shared signature registry out of `Store` and into
`Engine`.

This helps eliminate work that was performed whenever a `Module` was
instantiated into a `Store`.

Now a `Module` is registered with the shared signature registry upon creation,
storing the mapping from the module's signature index space to the shared index
space.

This also refactors the "frame info" registry into a general purpose "module
registry" that is used to look up trap information, signature information, and
(soon) stack map information.

crates/runtime/src/externref.rs

@@ -753,7 +753,7 @@ pub struct StackMapRegistry {
 struct StackMapRegistryInner {
     /// A map from the highest pc in a module, to its stack maps.
     ///
-    /// For details, see the comment above `GlobalFrameInfo::ranges`.
+    /// For details, see the comment above `GlobalModuleRegistry`.
     ranges: BTreeMap<usize, ModuleStackMaps>,
 }