add option to convert MassActionJumps to ConstantRateJumps

[isaacsas]

In some preliminary benchmarking for a talk I'm finding that when using Direct with a small number of jumps (~6), we can actually now get a ~30% speedup using ConstantRateJumps. This needs more investigation, but it may be that in the case of a small number of jumps, where we don't need function wrappers and use recursion for type-stability, we can now do better than MassActionJumps.