1.0.0 release

[taiki-e]

Tracking issue for the 1.0 release.

Blockers:

• stabilization of cfg(version) Tracking issue for RFC 2523, #[cfg(version(..))] rust-lang/rust#64796

TODO:

Once blockers resolved, do the following things:

• implement Remove const_fn(nightly) in favor of custom cfgs #28 "Remove const_fn(nightly) in favor of custom cfgs"
• add more docs (consistency of behavior with cfg(version) (Detect -Z assume-incomplete-release flag #35), etc.)