Add thread safety to default random

[ysbaddaden]

See #16157 and its comments for more details.

• Add Random.default thread local:
  • Replaces all usages of Random::DEFAULT
  • Fix: thread safety of Enumerable#sample(n) (split thread local)
  • Fix: thread safety of Crystal::System::File.mktemp (resolve thread local on each attempt)
• Remove most usages of Random.default to internalize its usage:
  • Add Random.next_int and Random.next_bool methods that can be called directly;
  • Prefer Random::Secure.random_bytes;
  • Methods taking a Random now default to random = nil
  • Docs are fixed to state that a custom Random instance can be passed for specific control.
• Deprecate now unused Random::DEFAULT
• Fix: deprecated Random::DEFAULT warning on Process.new on UNIX

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

Closes #16157.
Depends on #16173.

[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 😁