Adjust output of (*Message).String()
Implement basic GVariant parsing

Currently implemented:
 - Lexing and parsing basic types, arrays, variants and dicts
 - Type annotations / keywords
 - Simple type inference

TBD:
 - Bytestrings
 - Tuples
 - Dictionary entry sytax

Things that probably won't be implemented:
 - Maybe types
 - Positional parameters