Add ProtoTypeMaskRegistry, ProtoTypeMask, and FieldPath classes. The ProtoTypeMaskRegistry has functions that can be used to validate the input field masks and to check whether a field is visible.

Add ProtoTypeMaskRegistry, ProtoTypeMask, and FieldPath classes. The ProtoTypeMaskRegistry has functions that can be used to validate the input field masks and to check whether a field is visible. #91