Make BitsetHandler variable width and a pandas extension (#1448)

* Minimum viable pandas extension type. Still a lot of work to do

* Get base pandas DtypeExtension tests working

* Write __add__ method for union, to prove that we can overwrite methods

* Add typehints and clean up now out of date docstrings

* Add subtraction method

* Docstring with information divulge on nuiances with single-element accesses

* Return singletons as sets, which allows us to delegate methods on singletons to the set class

* Add remaining set-delegatable methods and implementations

* Additional docstrings and reorder definition of methods for sortability

* Add comparison operators

* Add comparison operations. Encounter bug with odd casting when passing sets to and and or

* linting pass

* Remove empty file

* Ensure that number of elements is correct if explicitly constructin a bitset series

* Fix some logic sequences and oddly-named variables

* NodeType -> ElementType, and remove now-redundant variable

* Fix logic in construct_from_string

* Fix weird casting bug when &= or |= assigning sets of multiple elements

* Write tests for __or__ and __and__. Breakout common fixtures into subfolder conftest.

* Refactor operation test format into parametrisation-only

* Write tests for series-wide comparison opps with 'scalars'

* Apply suggestions from code review

* Add pylint ignore to fix CI upstream issue