Revert the main payload of "Make `drop` and `dropWhile` fuse (#18964)"
This reverts the bits affecting fusion of drop
and dropWhile
of commit
0f7588b5 and keeps just the small refactoring
unifying flipSeqTake
and flipSeqScanl'
into flipSeq
.
It also adds a new test for #23021 (closed) (which was the reason for reverting) as well as adds a clarifying comment to T18964.
Fixes #23021 (closed), unfixes #18964.