WIP: Store a default_rng const which can be changed if the default RNG c...

[jperla]

...hanges in the future

Based on a comment here: #6018

This makes the default RNG a little more explicit, and more easily reversible if we use a different one in later versions.

This slows down random number generation.

deps/random/randmtzig.c might need to change as well

[JeffBezanson]

MersenneTwister isn't just a tag; it's a type of object that holds an actual RNG state.

[ViralBShah]

Whenever we want to change the default RNG in the future, we will do it as part of a major number bump and document it. We have worked really hard on faster random number generation, and we should be trying to get even faster.