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/trampoline/create_handle.rs

@@ -12,7 +12,7 @@ use wasmtime_runtime::{Imports, InstanceHandle, VMFunctionBody};
 
 pub(crate) fn create_handle(
     module: Module,
-    signature_registry: Option<&Store>,
+    store: Option<&Store>,
     finished_functions: PrimaryMap<DefinedFuncIndex, *const VMFunctionBody>,
     state: Box<dyn Any>,
 ) -> Result<InstanceHandle> {
@@ -26,12 +26,12 @@ pub(crate) fn create_handle(
     let data_initializers = Vec::new();
 
     // Compute indices into the shared signature table.
-    let signatures = signature_registry
-        .map(|signature_registry| {
+    let signatures = store
+        .map(|store| {
             module
                 .signatures
                 .values()
-                .map(|sig| signature_registry.register_wasmtime_signature(sig))
+                .map(|sig| store.compiler().signatures().register(sig))
                 .collect::<PrimaryMap<_, _>>()
         })
         .unwrap_or_else(PrimaryMap::new);