1.0.0 release

[taiki-e]

Tracking issue for the 1.0 release.

Blockers:

• [ ] stabilization of cfg(version) https://github.com/rust-lang/rust/issues/64796

TODO:

Once blockers resolved, do the following things:

• [ ] implement #28 "Remove const_fn(nightly) in favor of custom cfgs"
• [ ] add more docs (consistency of behavior with cfg(version) (#35), etc.)