Add C jumpahead functions for Cong, SHR3, MWC1, MWC2, LFSR113, LFSR88.
Add cxxtest unit tests for these jumpahead functions.

The jumpahead function implementations for Cong, MWC1, MWC2 require uint64_t.
In future it may be possible to do alternative implementations for platforms
without uint64_t.