feature: KVCell data strcture

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.