<random>: Implement size optimization for mersenne_twister_engine

[StephanTLavavej]

Boost's Mersenne Twister is half the size of ours because (as I remember hearing long ago) they implemented an optimization that saves space without significantly impacting time.

Interestingly, I observe that libstdc++ and libc++ appear to share this optimization, but only for their mt19937_64:

https://godbolt.org/z/8WKb1nWWW

vNext note: Resolving this issue will require breaking binary compatibility. We won't be able to accept pull requests for this issue until the vNext branch is available. See #169 for more information.