Make RandomState a lazy singleton. Fixes #27243

[cristicbz]

See #27243. The trick for single initialization is borrowed from lazy_static.

Creating and dropping many hash maps bench results:

before:
test collections::hash::bench::new_drop          ... bench:         102 ns/iter (+/- 5)

after:
test collections::hash::bench::new_drop          ... bench:           3 ns/iter (+/- 0)

[rust-highfive]

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[cristicbz]

cc @gankro since he's the one who filed the issue (and offered to mentor)

[cristicbz]

Ah, so I completely missed @apasel422's #27246, d'oh... Well, take this as an alternative implementation for the per-instance option with what seem to be better benchmarks (comparable to the thread_local) one.

Sorry about that...