Rate-control

-----

A ``RuntimeError`` is now raised when entering context managers more than once,
for objects for which it would cause undefined behavior.
Impacted classes are the following:

* Buckets:

* ``FixedWindowCounter``

* ``LeakyBucket``

* ``SlidingWindowLog``

* ``BucketGroup``

* Rate Controllers (not considering the underlying buckets):

* ``Scheduler``

The ``__slots__`` attribute was removed from all buckets and rate controllers,
for `practical reasons <https://stackoverflow.com/a/53089249>`_.

-----

* Added the ``NoopController`` controller, that simply lets requests through.

* Improved the performance of the ``Scheduler`` by better
detecting availability conditions for queued requests,
thus no longer requiring an internal lock.

-----

* **Breaking change**: Mixin helper methods were moved away from the ``RateController`` abstract class.

* **Breaking change**: Now define ``__slots__`` in all classes for better memory management

-----

* Allow passing kwargs in abstract ``RateController.request(...)``.

-----

* Now allow passing no bucket argument to the rate controllers.

* **Breaking change**: Removed the now obsolete ``UnlimitedBucket``.

-----

Breaking changes
^^^^^^^^^^^^^^^^

* Unified the API for the rate controllers.

* Now both the ``RateLimiter`` and the ``Scheduler`` use the following signature:
``async with controller.request(tokens): ...``

* The ``RateLimiter`` can now also be instantiated in an ``async with RateLimiter(...)`` statement.

* Rate controllers now manage their bucket's context in their ``async with`` statement,
so that we don't need to enter manually the bucket's context and then the rate controller's context.
This behavior can be disabled using the ``should_enter_context`` flag in the constructor.

* The ``rate_control.buckets`` module is now private.
The ``RateController`` base class, and all buckets,
are now exported in the main ``rate_control`` module.

New Features
^^^^^^^^^^^^

* The ``BucketGroup`` class now implements the ``Iterable[Bucket]`` protocol.