Common RNG API?

[jxy]

If we are to think of it from bottom up, a common RNG API would suit us well. This is what I was thinking when I put up sitmo. We may want the following:

STATE  →  STATE & RANDOM BITS
RANDOM BITS  →  floats
floats  →  distributions

And data Parallel versions of those. I loosely fashioned those in QEX/rng.

Another question would be whether we want vectorize RNG.

This is not really a top priority, but I guess it is good to put an issue here so we can pool our ideas together here.