Add OsRandom::choose for callers that want a uniformly selected reference from
any non-empty slice. This covers the common generic selection use case without
adding collection algorithms such as shuffle.
Keep OsRandom::pick as the byte-oriented helper by delegating it through
choose. This preserves the existing public API while sharing the usize range
sampler and avoiding the old u32 length limit.
Document choose, pick, and string_from with working examples, and add a choices
example that demonstrates generic slice selection alongside byte alphabets.
Test Plan:
- cargo test
- cargo clippy
- cargo clippy --benches
- cargo clippy --tests
- cargo +nightly fmt
Refs: IDEAS.md ergonomics backlog
