Initial, experimental proto support in Streamy.

What works:
* Single proto file schema input.
* Strings, int32/int64s, and message types.
* Repeated fields.
* [Un]marshalling by tag number.

What doesn't (including but not limited to):
* Service definitions.
* Imports.
* Enums.
* Libraries.
Unfortunately this will copy your TODO into every generated Dart. Can you move it to emitter.dart?