KVCell contains an optional value and provides access by key in two
flavors:
fn take_or_drop(&mut self, key: &K) -> Option<T>
fn take_or_keep(&mut self, key: &K) -> Option<T>Both return Some value only if provided key matches the contained one.
The behavior differs when the provided key doesn't match:
take_or_drop always leaves the cell empty. It supersedes CachedCell and copies its behavior.
take_or_keep retains contained value. It's a replacement for JointStateLatch.