merge the two versions of randmtzig_{randn,exprnd}

Those functions are now written without `@eval`: I guess this was used to work around a missing `rand(::MersenneTwister)` method, but I may have missed something (it seems to have been written in  1cbb6d0d672afd977e51b0f104a3d486943a7bf2, by @andreasnoack: can you confirm this change is OK?).

My understanding is that the speed improvements come from that the global RNG is accessed only once (as an argument of the function), instead of few times in `randmtzig_randn` (via `rand()`).