Grscheller.datastructures

* Major refactoring of SplitEnd class
* splitends is now a subpackage containing the se module
* removed supporting classes
* can now be empty
* grscheller.datastructures no longer uses nada as a sentinel value
* replaced with grscheller.fp.nothingness.noValue
* made classes in nodes module less passive with better encapsulation
* compatible with:
* grscheller.fp >= 1.0.0 < 1.01
* grscheller.circular-array >= 3.6.1 < 3.7

* Does not work with either
* grscheller.fp 0.3.3 (version working for 0.21.0 release)
* grscheller.fp 0.4.0 (latest version of fp)
* Needs an upgrade
* TODO: replace Nada with MB as was done for grscheller.fp.iterables

* renamed module `split_ends` -> `stacks`

* got back to a state maintainer is happy with
* many dependencies needed updating first

* works with all the current versions of fp and circular-array
* preparing for PyPI 0.21.0 release

* updated to use grscheller.fp.nada instead of grscheller.untyped.nothing
* made debugging tons easier
* updated to use all latest PyPI versions of dependencies
* three failed tests involving class SplitEnd
* putting off PyPI v1.0.0 release indefinitely
* all dependencies need to be at v1.0+
* need to work out SplitEnd bugs
* still need to finalize design (need to use it!)
* need to find good SplitEnd use case
* other Stack variants like SplintEnd??? (shared data/node variants?)