Get to 100% coverage, add tests for TaskSeq.zip, mapi, iter, exactlyOne and async variants

[abelbraaksma]

GOAL: 100% coverage!!! ✅

That is: 100% coverage for the BL parts of the code: all of TaskSeq.fs and TaskSeqInternal.fs.

This adds missing tests for the following functions

TaskSeq.zip
TaskSeq.exactlyOne
TaskSeq.tryExactlyOne
TaskSeq.map*
TaskSeq.mapi*
TaskSeq.mapAsync
TaskSeq.mapiAsync
TaskSeq.iter*
TaskSeq.iteri*
TaskSeq.iterAsync
TaskSeq.iteriAsync
TaskSeq.empty*
TaskSeq.collect*
TaskSeq.collectSeq*
TaskSeq.collectAsync
TaskSeq.collectSeqAsync
TaskSeq.ofResizeArray

* these had existing tests that I've improved a bit on, most notably, adding mutable-data tests

[abelbraaksma]

Turned out that TaskSeq.zip had a nasty little bug. Well, that's why we have tests. Fixed now.

Also, this made me think how zip should behave for lazy sequences. See #32 for a follow-up.

[abelbraaksma]

💚 LGTM: we reached 100% coverage!!!!