Minor refactoring.
Add tab completion for constants and namespaced ids.

This does a bunch of cool things:
  Ap<TAB> => Apricot
  Apricot::Id<TAB> => Apricot::Identifier
  Apricot::Core/mac<TAB> => Apricot::Core/macroexpand
  Math/sq<TAB> => Math/sqrt

So far it works flawlessly.
Add Hash#to_seq.
Add Set#to_seq.