Add traits for applying trailing decl docs
Use trait application for trailing decl docs

Using HOW.set_trailing_docs works for classes, but it
breaks down for routines because they all share a single
HOW, resulting in every Routine sharing the same trailing
docs
Track line numbers for routine declarations

This is necessary to implement trailing declarative docs
for routines
Track routines as potentials for trailing decl docs
Consider method definitions for trailing decl docs