Resolves #4661.

To support this we need to change the algebra of ZIO slightly. lock describes a static scope of shifting onto and off of a thread pool around a given effect. However, that doesn't give us the power to describe shifting for dynamic scopes. To support this we remove lock as a primitive and replace it with shift, which shifts to a specified Executor. lock can then be implemented using a combination of shift and bracket. With this change, lock just becomes a ZManaged effect that describes shifting onto a thread pool as its acquire effect and shifting back as its release effect.