Update repository URLs to use underscore instead of hyphen

Add `#![deny(missing_docs)]` across multiple crates and fill in
documentation for all public items, including struct fields, enum
variants, trait methods, and proc macros.  Also mark two shell scripts
as executable.

Remove the `Default` bound from `pick()` and `EntryPicker::pick()`,
allowing non-Default types to be used as arguments.

Add `IntoPicker::pick_or`, `pick_or_default`, and `pick_or_route`
convenience methods for common fallback patterns.

Fix token splitting in `arg!` macro to correctly handle angle brackets.