Factor choice of rand out of Distribution

[julianmichael]

It can be useful for the programmer to be able to specify a seed.
This isn't possible if ThreadLocalRandom is used as a private val
inside of Distribution. I factored it out by parametrizing Distribution
over rand and providing a default that is equivalent to the original.

The only incompatilibity this should introduce is with the change of
freeze. Because it creates a distribution (i.e., uses rand) its
natural place seems to be in the new Distributions class.
Otherwise a programmer could accidentally introduce seemingly
nondeterministic behavior even with a preset seed.

[jliszka]

Looks great, thanks for the contribution!

Was it necessary to move freeze out to the companion in order to make this work? Lots of other methods in Distribution create new distributions (map for example), so I wouldn't necessarily call that a criteria for moving it.

[jliszka]

Oh I see. If you create your own instance of Distributions with a preset seed, you might accidentally revert back to the one used by object Distribution because freeze refers to it explicitly. Makes sense.