Add Sync::ConditionVariable

[ysbaddaden]

Imports Sync::CV, Sync::Lockable and Sync::ConditionVariable from the sync shard.

• Sync::CV is the foundational type for condition variables and interacts with Sync::Mu — internal, ported from the nsync library;

• Sync::ConditionVariable is the public type, meant to interact with Sync::Mutex, Sync::RWLock, ...

• Sync::Lockable is to allow to use Sync::ConditionVariable with any lock that internally uses Sync::MU (Sync::Mutex, Sync::RWLock, ...);

Follow up to #16399