SI-6196 - Set should implement filter

Implements a version of filter and filterNot that reuses as much as
possible from the existing tree instead of building an entirely new one
like the builder-based filter does. This results in significant performance
improvements on average.

Adds a test of basic correctness of filter and filterNot as well as of the
desirable properties of the new filter implementation.

This is a collaboration between me and @Ichoran

Conflicts:
	src/library/scala/collection/immutable/HashSet.scala
SI-6200 - HashMap should implement filter

This is the exact same algorithm as in SI-6196, with only slight
differences due to the two type arguments of HashMap.

Filter is tested by the new comparative collection test by @Ichoran, but
there is nevertheless a small correctness test in t6200 in addition to
tests of the new desirable behavior.