Add thread safety to default random

[ysbaddaden]

See #16157 and its comments for more details.

• Add Random.default thread local to replace all usages of Random::DEFAULT.
• Fix thread safety of Enumerable#sample(n) by splitting the thread local to a new random sequence.
• Deprecate now unused Random::DEFAULT.

To be defined: we might want to keep Random.default undocumented 🤔

Depends on:

• Add thread_local macro #16173
• Refactor internal Crystal::System::Process#fork on UNIX #16191
• Add Random.next_bool and .next_int #16297
• Prefer Random::Secure.random_bytes #16298
• Set default random arg to nil instead of Random::DEFAULT #16299

Closes #16157.

[straight-shoota]

We should probably extract the refactor of Crystal::System::Process.fork. This change is not directly related to the thread safety of random.
And we need it for other reasons as well. For example, we might want to use clone(2) instead of fork for spawning a new process in order to get an atomic pidfd.

[ysbaddaden]

Or move to posix_spawn (and pidfd_spawn on Linux) instead 😁

[straight-shoota]

This is quite a big patch. We should probably split it down a bit more into smaller, self-contained change sets.

The change for internalizing Random.default could each already be an individual PR:

• Change method signatures to random = nil and update docs
• Add Random.next_int and Random.next_bool
• Prefer Random::Secure.random_bytes

[ysbaddaden]

Rebased from master with extracted commits squashed back in.

[straight-shoota]

There's a leftover DEFAULT reference in #next_int.

[ysbaddaden]

Extracted def from macro and undocumented Random.default for the time being.

We might want to formalize an API to "split a random" with a couple #split and #split! methods, though. For example xoshiro, splitmix and Java's SplittableRandom all have the ability to split. In that case we might want to extract yet-another PR 😅

[ysbaddaden]

Final rebase on master to bring the reworked and now official #split and #split_internal methods. That simplifies the macro as .split_on_stack only, since the whole behavior is now bundled into the split methods.

I renamed the TLS as Random.thread_default.