Move the Store::signature_cache field (#847)

This commit removes the `signature_cache` field from the `Store` type
and performs a few internal changes which are aimed to be a bit forward
looking towards #777, making `Store` threadsafe.

The changes made here are:

* The `SignatureRegistry` internal type now contains the reverse map
  that `signature_cache` was serving to do. This is populated on calls
  to `register` automatically and is accompanied by a `lookup` method as
  well.

* The `register_wasmtime_signature` and `lookup_wasmtime_signature`
  methods were removed from `Store` and now instead work by using the
  `Compiler::signatures` field.

* The `SignatureRegistry` type was updated to have interior mutability.
  The global `Compiler` type is highly likely to get shared across many
  threads through `Store`, so it needs some form of lock somewhere for
  mutation of the registry of signatures and this commit opts to put it
  inside `SignatureRegistry` which will eventually allow for the removal
  of most `&mut self` method on `Compiler`.

crates/api/src/module.rs

@@ -401,11 +401,5 @@ fn compile(store: &Store, binary: &[u8], module_name: Option<&str>) -> Result<Co
         module_name,
         store.engine().config().debug_info,
     )?;
-
-    // Register all module signatures
-    for signature in compiled_module.module().signatures.values() {
-        store.register_wasmtime_signature(signature);
-    }
-
     Ok(compiled_module)
 }