Add OsRandom::fill_bytes so callers can fill a byte slice without importing
std::io::Read. This keeps the common raw-byte use case close to the rest of
the crate's typed convenience API while still forwarding to the same
/dev/urandom handle.
Implement Default for OsRandom as the infallible convenience constructor. The
fallible OsRandom::try_new API remains the right choice when the caller wants
to handle an unavailable operating-system randomness source explicitly.
Document the public constructors and byte-fill helper with working examples,
and add a runnable bytes example for the top-level workflow.
Test Plan:
- cargo test
- cargo clippy
- cargo clippy --benches
- cargo clippy --tests
- cargo +nightly fmt
Refs: IDEAS.md ergonomics backlog
