std::rand: add MT19937 and MT19937-64. (Mersenne Twister RNG)

Implementations of the 32- and 64-bit variants of the Mersenne Twister
RNG algorithm. C++11 mandates "mersenne_twister_engine" in its random
number library, so this is just imitating that.

(cc https://github.com/mozilla/rust/pull/9810, separate because that PR is large enough and this is a self-contained change.)