ENH: Rename default_gen -> default_rng

[rkern]

Per request for a more evocative name. default_generator was also considered, but some felt it too long. And in any case, it's the RNG algorithm that's the default, not the Generator itself. This might avoid some confusion in case people expect default_generator to return the default instance of Generator (akin to the default RandomState instance) that we don't have.

[charris]

Thanks Robert.