ThreadUnsafeRandom is a random implementation that is
identical to LegacyRandomSource behaviourally, but
without the thread checks.
SimpleThreadUnsafeRandom is ThreadUnsafeRandom except with
its nextInt(int) function replaced with a faster
but more biased implementation when bound is very large.
Additionally, replace Level/Entity randoms with ThreadUnsafeRandom.
This avoids the expensive CAS logic at the expense of losing the
thread check.
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