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/instance.rs

@@ -186,7 +186,7 @@ impl Instance {
                 // HACK ensure all handles, instantiated outside Store, present in
                 // the store's SignatureRegistry, e.g. WASI instances that are
                 // imported into this store using the from_handle() method.
-                let _ = store.register_wasmtime_signature(signature);
+                store.compiler().signatures().register(signature);
             }
 
             // We should support everything supported by wasmtime_runtime, or