New mutable hash map with Long keys: partially solves SI-5263 and is relevant
to SI-6825.

The hash map is based on an open addressing scheme that provides
dramatically faster (mostly due to specialization) get and contains
operations than either the standard Java HashMap or any of the existing
Scala hash maps.  It doesn't work well above 500,000,000 elements as
the arrays cannot scale past 2^30 elements.  Maps are not faster in
general due to the lack of specialization of Function1[Long, V].
New AnyRefMap fixes SI-5263 to the extent practical.

An open addressing hash map based on a similar scheme to mutable.LongMap.
It delivers performance equivalent to Java's HashMap for pretty much all
AnyRefs, plus it works nicely with Scala's collections.