Implement Random core class and random functions in Kernel

[lopopolo]

This commit implements all APIs of Random except for the internal
private method Random#state and Random::Formatter.

Random is implemented with the rand crate's not cryptographically
secure SmallRng. For cryptographically secure APIs like
Random::urandom, the re-seeding rand::thread_rng is used.

Random is gated behind a crate feature.

To support the Random::DEFAULT interpreter-widre PRNG, the interpreter
stores a Random on the State and injects a special backend to allow
the DEFAULT constant to target this State-owned Random.

Random is implemented with multiple backends and has a small API which
would make supporting a future CRuby algorithm possible. The Rand
backend is generic over the rand::Rng implementation.

This commit passes 30 ruby/specs and fails 14:

$ ./scripts/spec.rb artichoke core random
Passed 30, skipped 0, not implemented 0, failed 14 specs.

The specs that fail fall into the following categories:

• Core class not implemented: Bignum, Struct, Complex, Rational.
• Private methods not implemented.
• PRNG is not the same implementation as the one in CRuby.
• Encoding not implemented, particularly Encoding::ASCII-8BIT.

This PR does some small refactorings:

• Implement Value::implicitly_convert_to_int to simplify arg handling
  in random trampoline. All of Artichoke is not fully migrated.
• Move Float type alias from float converter to types module.

This PR implements the Random core class and Kernel random
functions. This PR is a partial implementation of GH-34.