Add skipWhile, skipWhileInclusive with async variants, and takeUntil etc

[abelbraaksma]

Similar to #122. The naming should be skipWhile, even though skipUntil was proposed as well in that thread. F# Core has skipWhile, so we should stick with that. Also, I think they are semantically different:

Skip while

• skipWhile: as long as predicate is true, keep skipping, then stop
• skipWhileAsync: same, but async
• skipWhileInclusive: same, but yield first, then test predicate on yielded value
• skipWhileInclusiveAsync: same, but async

Skip until

• skipUntil: skip until predicate becomes true
• skipUntilAsync: same, but async
• skipUntilInclusive: same, but skip first, then test predicate
• skipUntilInclusiveAsync:same, but asyncate

And the missing Take Until

We should include these if we do the skipUntilXXX functions:

• takeUntil: as long as predicate is false, keep skipping, then stop
• takeUntilAsync: same, but async
• takeUntilInclusive: same, but yield first, then test predicate on yielded value
• takeUntilInclusiveAsync: same, but async

Not sure we should have both until and while, as they are logically each other's opposites.

[abelbraaksma]

Updated the body of the request to include the async variants and the takeUntil variants as well to complete the block.

[abelbraaksma]

Note on #183: we (@bartelink and I, well, he convinced me) decided to skip the until variants of these functions, as they were nothing more than the logical inverse (i.e., users can use the not function to reach that functionality).

Further work is through #208 and #219.