Improved custom builder lookup (solves #6)

This implementation uses auxilary macro.
@kciesielski The issue with c.typecheck (and c.inferImplicitValue that's essentially based on c.typecheck) in macro annotations is a yet unsolved problem in the design of macro annotations. That's indeed a bug.

@mkubala Your high-level hypothesis about the reason for this limitation is spot-on (if you're interested in details, https://github.com/scalamacros/paradise/issues/14 provides a lot of information on the nature of the issue).