Merge pull request #296 from matklad/matklad/plfix

release 1.21.4

Merge pull request #295 from matklad/matklad/plfix

Merge pull request #290 from alexanderkjall/run-tests-with-no-default-features

Merge pull request #292 from phil-opp/doc-fix

fix OnceCell::wait with features=parking_lot

Fix: `sync::OnceCell::try_insert` docs should import `sync` variant

make the command 'cargo test --no-default-features' work

Merge pull request #287 from AbeZbm/add-tests

Add tests for OnceRef

Merge pull request #286 from briansmith/b/once-ref-dry

Prepare for 1.21.3 release.

DRY `race::OnceRef::{get_or_try_init, set}`.

Merge pull request #285 from briansmith/b/once-ref-cold

Mark initialization of `OnceRef::get_or_try_init` cold.