Clean up a few things

[KodrAus]

This is a list of a few things that should be cleaned up:

• use fully qualified paths for Box and Fn traits in generated code
• support mutable and by-value receivers for Box
• properly reject by-value receivers for &mut
• support associated methods (where there's no self receiver)
• clean up the attribute parser and make sure it works when there's no whitespace (the indexing right now will probably break without whitespace padding on things)