Grscheller.fp

* New module `grscheller.fp.nothingness` for
* Singleton `noValue` representing a missing value
* similar to `None` but while
* `None` represent "returned no values"
* `noValue: _NoValue = _NoValue()` represents an absent value
* mostly used as an implementation detail
* allows client code to use `None` as a sentinel value
* prefer class `MB` to represent a missing value in client code

* API change for fp.iterables
* function name changes
* `foldL`, `foldR`, `foldLsc`, `foldRsc`
* `sc` stands for "short circuit"
* all now return class woException.MB

* added more functions to fp.iterables module
* take(it: Iterable[D], n: int) -> Iterator[D]
* takeWhile(it: Iterable[D], pred: Callable[[D], bool]) -> Iterator[D]
* drop(it: Iterable[D], n: int) -> Iterator[D]
* dropWhile(it: Iterable[D], pred: Callable[[D], bool]) -> Iterator[D]

* fp.iterables `foldL_sc` & `foldR_sc` now have
* common paradigm
* similar signatures

* added fp.iterables function `foldR_sc`
* shortcut version of `foldR`
* not fully tested
* docstring not updated

* added fp.iterables function `foldL_sc`
* shortcut version of foldL