Implement jump multiplier calculation. Add const where appropriate.

The LehmerSet class has jump multipliers hard coded for 128, 256, 512,
and 1024 streams. For simple pRNG needs, these should be sufficient.

For particularly pRNG-hungry simulations, an arbitrary number of
streams can be requested. A jump multiplier can be calculated offline
(recommended, but not necessary), or the jump multiplier can be
calculated at runtime. Some type trickery is used to allow efficient
modular exponentiation in 32 bit systems (int_least64_t should be
defined on any reasonable compiler).

The const qualifier was added everywhere I felt it appropriate.