unprivates important helpers in Namers.scala

This is the first of two commits that enable hooks necessary to implement
macro annotations in an honest, hackless compiler plugin.

This particular commit turns certain helpers into public methods. Of course,
there is a probability that with the evolution of macro paradise, I will need
more helper methods, and those will have to be called via reflection, but
at least for now it's nice to have a reflection-less plugin :)
hooks for naming and synthesis in Namers.scala and Typers.scala

Interestingly enough, despite of the implementation surface being
quite noticeable, it is enough to hijack just `enterSym` and typechecking
of stats for packages, templates and blocks in order to enable macro annotations.
That and `ensureCompanionObject`, which I couldn't abstract away so far.