Send/Sync bound needed on V in `impl Send/Sync for ARCache<K, V>`

Affected versions of this crate unconditionally implemented Send/Sync traits for ARCache<K, V> type.

This allows users to send/access types that do not implement Send/Sync, which can cause a data race.

The flaw was corrected in the 0.2.6 release by adding bounds K: Send + Sync & V: Send + Sync to affected Send/Sync trait implementations.