kills introduceTopLevel

As we've figured out from the practice, introduceTopLevel is seductively
useful but unfortunately not robust, potentially bringing compilation
order problems.

Therefore, as discussed, I'm removing it from the public macro API.
Alternatives are either: 1) delve into internals and use BrowsingLoaders,
2) use macro paradise and experiment with macro annotations:
http://docs.scala-lang.org/overviews/macros/annotations.html.