ConstantRNG should implement UniformRandomBitGenerator
ConstantRNG should implement UniformRandomBitGenerator

Summary: As per http://en.cppreference.com/w/cpp/concept/UniformRandomBitGenerator it was almost there, except that `min()` and `max()` need to be static members.

Reviewed By: yfeldblum

Differential Revision: D3507595

fbshipit-source-id: 5e0b321c477e37ab8a3487ad643caa83cc6cfc9d

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