Define a proper hash table library

Define a proper hash table library

Define a proper hash table library, instead of the current ad hoc stuff
used for both labels and macros.  This only implements the actual
library; it is not yet used.

We use a CRC64 as a prehash.  This is almost certainly overkill,
although it is rather efficient (except, arguably, the table lookup)
on 64-bit platforms, and not all that bad on 32-bit platforms.  All we
really need is a function which produces two independent 32-bit
results which are used as the primary and secondary hash
respectively.  Either way, the prehash function is easily replacable
if/when we have a quicker alternative.