Port ClassfileParser improvements from Scala 2

[smarter]

Our version of ClassfileParser was copied from Scala 2 a long time ago, since then we've ported some of the improvements made there but not all of them, in particular the recently merged scala/scala#6653 (comment) looks interesting.

[anatoliykmetyuk]

After talking to @adriaanm, I opened the scala/scala-dev#625 issue to see if we can address the problem of code sharing in a more generalised way.