Add TaskSeq.tryitem, item, tryLast, last, tryHead, head, tryPick, pick, tryFind, find, choose, filter, isEmpty and more for IAsyncEnumerable<'T>

[abelbraaksma]

First batch of the functions mentioned in #22, see title.

Implemented the following (WIP):

• isEmpty
• tryExacltyOne
• exactlyOne
• head
• tryHead
• last
• tryLast
• item
• tryItem
• filter
• filterAsync
• choose
• chooseAsync
• pick
• pickAsync
• tryPick
• tryPickAsync
• find
• tryFind
• findAsync
• tryFindAsync

The signatures (WIP, will update) completed:

val tryHead: taskSeq: taskSeq<'T> -> Task<'T option>
val head: taskSeq: taskSeq<'T> -> Task<'T>

val tryLast: taskSeq: taskSeq<'T> -> Task<'T option>
val last: taskSeq: taskSeq<'T> -> Task<'T>  // may raise

val tryItem: index: int -> taskSeq: taskSeq<'T> -> Task<'T option>
val item: index: int -> taskSeq: taskSeq<'T> -> Task<'T>  // may raise

val tryExactlyOne: source: taskSeq<'T> -> Task<'T option>
val exactlyOne: source: taskSeq<'T> -> Task<'T>  // may raise

val choose: chooser: ('T -> 'U option) -> source: taskSeq<'T> -> taskSeq<'U>
val chooseAsync: chooser: ('T -> #Task<'U option>) -> source: taskSeq<'T> -> taskSeq<'U>
val filter: predicate: ('T -> bool) -> source: taskSeq<'T> -> taskSeq<'T>
val filterAsync: predicate: ('T -> #Task<bool>) -> source: taskSeq<'T> -> taskSeq<'T>

val tryPick: chooser: ('T -> 'U option) -> source: taskSeq<'T> -> Task<'U option>
val tryPickAsync: chooser: ('T -> #Task<'U option>) -> source: taskSeq<'T> -> Task<'U option>
val pick: chooser: ('T -> 'U option) -> source: taskSeq<'T> -> Task<'U>  // may raise
val pickAsync: chooser: ('T -> #Task<'U option>) -> source: taskSeq<'T> -> Task<'U>  // may raise

val tryFind: predicate: ('T -> bool) -> source: taskSeq<'T> -> Task<'T option>
val tryFindAsync: predicate: ('T -> #Task<bool>) -> source: taskSeq<'T> -> Task<'T option>
val find: predicate: ('T -> bool) -> source: taskSeq<'T> -> Task<'T>  // may raise
val findAsync: predicate: ('T -> #Task<bool>) -> source: taskSeq<'T> -> Task<'T>  // may raise

[abelbraaksma]

Important note: I have disabled parallel test runs due to a race condition that was hard to pin down. Dozens of commits and reverts have been squashed back into one, to keep a clean history.

See #25 for further investigation.