[mypyc] Add librt.random module (#21433)
The stdlib `random` module is fairly often used in performance critical
code, and it's not super efficient. Add `librt.random` with a subset of
the stdlib module interface that is optimized for performance when
compiled.
Use ChaCha8 as the algorithm. Based on some research, this is a modern,
high-quality PRNG algorithm. It's used by Go `math/rand/v2`, among
others.
This is a non-cryptographic PRNG, similar to the stdlib `random` module
(but this uses a different algorithm).
I used Claude Code and Codex to write all the code, but I iterated on it
quite a lot and did a bunch of manual validation and code review. I also
asked Codex to explicitly check that the ChaCha8 implementation is
correct by comparing it to a reference implementation.
Use thread-local RNG state for module-level functions to enable good
scaling in free-threaded builds. There's some extra complexity from
having to free the state at thread exit.
I asked a LLM to generate and run a benchmark, and here are the results
on 3.14:
```
│ Function │ vs stdlib(compiled) │ vs stdlib(interpreted) │
│ random() │ 3.2x │ 4.8x │
│ randint() │ 16.6x │ 18.0x │
│ randrange() │ 14.5x │ 16.2x │
│ choice() │ 12.9x │ 10.3x │
```
`choice()` was replaced with `randrange` when using `librt`, since we don't provide it as part of this fairly minimal API.
