ENH: Request is now a traitlets-based class.
REF: rename clause->rule modules.
FEAT: add id_to_literal conversion to Pool.
ENH: PackageRule is now a traitlets.
FEAT: add RuleSet class.

This class may be superflous, but for now, we will just have a simple
version of it to mimic Composer design. Should be easy to scrap later
on.