- Add a env var to pass a set of expected prefixes for validation.
- Report warnings/errors based on the expected prefixes vs. the data in the files compiled.
- Use some helpers from common directory.

The previous two methods make it easy to transform between any and normal message.
unPackeTo will throw error if the type url in any doesn't match the type of the message to be transformed to.
is checks any's type url matches the give GPBMessage type.

- Style fixups in the code.
- map<> serialization fixes and more tests.
- Autocreation of map<> fields (to match repeated fields).
- @@protoc_insertion_point(global_scope|imports).
- Fixup proto2 syntax extension support.
- Move all startup code to +initialize so it happen on class usage and not app startup.
- Have generated headers use forward declarations and move imports into generated code, reduces what is need at compile time to speed up compiled and avoid pointless rippling of rebuilds.