BitSet fixes - assert on presence of 'ghost bits' and others

Fixes after reviewing org.apache.lucene.util.FixedBitSet, LongBitSet and corresponding tests:

• Some methods rely on the fact that no bits are set after numBits (what I call 'ghost' bits here).
  • cardinality, nextSetBit, intersects and others may yield wrong results
  • If ghost bits are present, they may become visible after ensureCapacity is called.
  • The tests deliberately create bitsets with ghost bits, but then do not detect these failures
• FixedBitSet.cardinality scans the complete backing array, even if only numWords are in use