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`.
e5af0ae3de