This class is a very simple bitset class. The DFA content model code needs
to support a bit set, but the java BitSet class is way, way overkill. Our
bitset never needs to be expanded after creation, hash itself, etc...
Since the vast majority of content models will never require more than 64
bits, and since allocation of anything in Java is expensive, this class
provides a hybrid implementation that uses two ints for instances that use
64 bits or fewer. It has a byte array reference member which will only be
used if more than 64 bits are required.
Note that the code that uses this class will never perform operations
on sets of different sizes, so that check does not have to be made here.