Performance idea: ILReader / type system

[dgrunwald]

ILReader is still somewhat expensive when resolving method references.

1. TypeProvider.GetTypeFromReference() still constructs a GetClassTypeReference and IModuleReference. These are unnecessary allocations and should be refactored away. Moreover, assemblies tend to have only few module references, so it's highly useful to cache the result of resolving them within the MetadataAssembly.

2. We currently fully resolve the signature of a memberref into type objects; then use them only for comparing the type objects against the available methods. It's possible to directly compare the signature bytes instead (SignatureBlobComparer). This might speed things up (though it might also cause us to resolve types slightly more often).

Also, SignatureBlobComparer.TypeHandleEquals needs adjustment. The current implementation is both inefficient (allocates strings, splits of backticks for generics), and ignores the assembly identity.

3. We might consider reintroducing caches for resolving typerefs and/or memberrefs (we had those in ILSpy 3.x). But let's first try to optimize the other ways; a fast robust IEntity<->memberref comparison will also help analyzer performance.