Migrate all abstractions to Synchronisation layer or to edge

[pitr-ch]

Classes using mutex:

• MVar
• TVar
• Dereferencable (when is it suppose to be removed?)

Classes with final ivars not using layer:

• Maybe
• Tick
• possibly others

[jdantonio]

Removing Dereferencable is a breaking API change. I think 2.0 is the appropriate time.

I've tried to update MVar to use the synchronization layer but the tests fail every time. I haven't explored it deeply.

Anything related to channels (like Tick) can wait. Those are Edge and and need a lot more work before I would recommend using them.