refactor(skip+takeWhile): Make impls consistent

[bartelink]

Calved from #220: Cleanup/refactoring of skipWhile and takeWhile.

Largely for me to distil the implementation so I could get to a clear understanding of the difference between the Inclusive and non-Inclusive variants.

[abelbraaksma]

I spent a little time today with the code changes proposed here. In general, I'm totally in favor of cleaner code. However, there's a (sometimes) significant performance penalty associated with these changes:

let inclusive =
    match whileKind with
    | Inclusive -> true
    | Exclusive -> false

match predicate with
| Predicate predicate -> // takeWhile(Inclusive)?
    while cont do
        if predicate e.Current then
            yield e.Current
            let! hasMore = e.MoveNextAsync()
            cont <- hasMore
        else
            if inclusive then  // this is a problem. This should be lifted
                yield e.Current

            cont <- false

The original code makes the decision between inclusive/exclusive on the outer rims of the function, ensuring that these static values do not trickle down to the inner loop.

However, I am not quite happy with the current implementation either, so I'll give it another round tomorrow, while also keeping the consistency-improvements that you added here.

[bartelink]

Thanks for all the investigation work Please don't invest too much time in this; I and the world would much value getting out of alpha above not 'losing' my work or any such concerns Ultimately the tests cover all this well, so the impl consistency is not truly critical

[abelbraaksma]

This PR has been continued and completed in #235, using the principles shown here. It turned out, a lot more could be simplified in these functions.