Changes:
* Coroutines have no logical context by default (a revert to the V3
semantics).
* All APIs have been placed to the ``contextvars`` module, and
the factory functions were changed to class constructors
(``ContextVar``, ``ExecutionContext``, and ``LogicalContext``).
* ContextVar.lookup() got renamed back to ContextVar.get() and gained
the `topmost` and `default` keyword arguments;
* Added ContextVar.delete().
* Fixed ContextVar.get() cache bug (thanks Nathaniel!).
* Added a few more design considerations points.
* Various minor edits and enchancements.
* pep-550: Rename LocalContext -> LogicalContext
* pep-550: Rename ContextItem -> ContextKey
* pep-550: Update ContextKey caching
* pep-550: Reorder Generators and Coroutines sections; drop await/ceval
* pep-550: Some rewording in the HAMT Performance section
* pep-550: Add a couple of paragraphs to Approach 3
* pep-550: Add version 3 summary of changes
* pep-550: A few more edits
* pep-550: Update the Version History section
Yury Selivanov
Elvis Pranskevichus
Yury Selivanov