support for `__slots__`

[jbrelwof]

CircuitPython does not use __slots__ within class definitions. Support for __slots__ could result in potentially significant runtime and memory savings, especially for classes with lots of instances (because no __dict__ per instance is needed).

Some of the benefits of __slots__ overlap with collections.namedTuple, but namedTuple instances are immutable. However, I suspect even a naive/limited __slots__ support implementation would probably be more complex than namedTuple.

[dhalbert]

This kind of change we mostly depend on MicroPython to do. There was some work on __slots__ there that was never merged:
micropython#3392
micropython#6070