More diagnostic info in SplayIntSet.
Added a still buggy BitIntSet.
Implementing a long as a small bit set first.
SplayNode optimisation.

Not explicitly storing the int set, if all integers in the range are
contained.
LongBitSet is working.
BitSet work in progress.
BitSet still a work-in-progress.
BitSet passes all tests.
Step towards integrating BitSet into SplayIntSet.
SplayNode dyn. switches to more mem. eff. repr.
Merge branch 'intbitset'