Miscellaneous performance fixes for Crossgen2 (#758)

Miscellaneous performance fixes for Crossgen2 (#758)

* Miscellaneous performance fixes.
Create comparison functions for Enum (Enum.CompareTo boxes since it takes an Object).
In ModuleToken, use the existing MetadataReader instead of making a new one each time.
Use Dictionary instead of ImmutableDictionary in CoreRTNameMangler to reduce allocations.
In CopiedFieldRvaNode, only read as much of the section as we need to copy.