Random default RNG alias plus stopgap rndGen and unpredictableSeed.

The latter two are implemented in a completely cracked way that
will hopefully be adequate for the purposes of testing functionality
that relies on their existence.
Beginning of std.random2.distribution module with uniform().

This is a rather cracked copy of the existing std.random.uniform.
Ideally it should not require too much tweaking to work effectively
with the new std.random2, but there are currently some issues with
unittests which have temporarily been commented out with
version(none) { ... }.