Base class that provides the implementation for RHexTree, which is just a thin template.

An instance of this class can have up to eight 16-way prefix trees, with heights from 1 to 8. All the values are stored in the leaves. To find a value from a 32-bit key, first the key is decomposed into 8 hexadecimal digits and then the prefix tree with height matching the number of digits in the key (ignoring zeros to the left) is traversed using the sequence of digits in the key as the indexing string. Offsets are internally used instead of pointers to allow instances to be placed in a heap shared between several processes.