Lint pass to flag unneeded qualified names

[catamorphism]

I noticed that in ty, there was some code that referred to ast::m_mutbl even though ty imports syntax::ast::*, so it could just be m_mutbl. There should be a lint pass to point out these cases so as to reduce the amount of random noise in code.

[catamorphism]

Still a good idea IMO; doesn't really fit into any of the milestones.