Add __slots__ to all types and nodes (#11197)

Quick review of changes:

* I had to move all class-level values into self. definitions, because class attributes and 
  incompatible with __slots__. This seems like a good idea for me, because there was no 
  reason for them to be class-level. I can even imagine several bugs based on over-use of 
  class-level attributes mutation. It might take more memory in some cases though.
* I've added several TODO items to op: str, it seems like Enum would be a better fit there

Closes #11195

Author: sobolevn