Implement reflection properly for fields.

- FieldAccessorTable is now non-generic
- We don't have a static field per message type in the umbrella class. (Message descriptors are accessed via the file descriptor.)
- Removed the "descriptor assigner" complication from the descriptor fixup; without extensions, we don't need it
- MapField implements IDictionary (more tests would be good...)
- RepeatedField implements IList (more tests would be good)
- Use expression trees to build accessors. (Will need to test this on various platforms... probably need a fallback strategy just using reflection directly.)
- Added FieldDescriptor.IsMap
- Added tests for reflection with generated messages

Changes to generated code coming in next commit.