Add token, hex, and pin helpers for the common ASCII string formats already
represented by the crate's built-in charsets. These methods keep frequent use
cases concise while continuing to route through string_from, so callers still
get the same uniform per-character sampling behavior.
Avoid adding a password preset in this change. Password generation implies
policy choices around symbols, ambiguous characters, and service-specific
constraints, while these presets are direct names for existing alphabets.
Document every preset with doctested examples and add a runnable presets
example. Update the demo to show both presets and custom string generation.
Test Plan:
- cargo test
- cargo clippy
- cargo clippy --benches
- cargo clippy --tests
- cargo +nightly fmt
Refs: IDEAS.md ergonomics backlog
