conquer-once 0.2.0

Synchronization primitives for lazy and one-time initialization
Documentation
# Releases

## Release `0.2.0`

- fixes internal soundness issues (e.g. one similar to
  https://github.com/rust-lang/rust/pull/65719)
- renames `OnceCell::new` to `OnceCell::uninit` and `OnceCell::initialized` to
  `OnceCell::new`
- adds some convenience trait implementations and tightens some trait bounds
  (breaking)
- improved internal code structure and code re-usage, better facilitates
  monomorphic code usage