Improve expression handling for watchpoints

[kpgriesser]

This is mentioned in sstsimulator#1452 and expanded here.

comment from @leekillough 👍

I would prefer to use C++ comparison functors like std::less, std::greater, std::equal rather than creating a separate ObjectMapComparison type, but that can be at a later release.

(Rev uses those functors to implement all of the different RISC-V comparisons.)

This is a good time to upgrade the comparison code. It would also be nice to enhance the comparison support to include things like grouping expressions in parentheses.