Minor Additions to IDL4 Support

[iguessthislldo]

• Support the following IDL v4 features:
  • Empty structs (Fixes #1254)
  • octet and wchar union discriminators (Fixes #1255)
  • Allow using typedefs of int8 and uint8 as union discriminators
  • Allow using empty parentheses in annotation applications to workaround syntax errors when an annotation with no arguments has to be followed by a complete scoped name: @example_annotation() ::ex::ExampleType
  • Reserve the bitfield, bitmask, and bitset keywords in IDL v4 (these are not implemented yet) (Fixes #1256)
• Allow using map as an identifier in IDL v3 again