Document randomSample alias
UniformRandomDeviceTypes TypeTuple of all uniform-RNG random devices

Use of this {OS,architecture}-dependent TypeTuple allows us to fix
a problem with the unittests, which would previously have failed to
compile on non-Posix systems.
Rename randomSample => sample

An alias has been provided for older code.  Some variable names in
the unittests have been tweaked to avoid confusion.