Issue Description:

It looks fairly sparse at the moment, and some fundamental APIs appear to be totally untested, for example .next(upperBound: ).

We should have at least some basic testing that the results are in the specified ranges and that no obvious modulo bias occurs.

There should probably also be some longer running tests that validate statistical properties more carefully, but those likely shouldn't be part of the basic test set (do we need to figure out where to put them, or is there already somewhere appropriate? We'll probably want to include tests for floating-point binary <-> decimal conversion and a few other things in whatever we do for longer-running checks).