Stylistic tweaks to function/class 'if' constraints.
std.random2.adaptor is inaugurated with RandomSample.

This class-based version of RandomSample, relying in turn on
class-based RNGs, should resolve the known issues with the
existing std.random.RandomSample.

It's also able to simplify things somewhat in terms of constructors
and unittests.  (And it has 100% test coverage:-)