proposal: x/exp/xiter: add IsSorted/IsSortedFunc

[leaxoy]

Proposal Details

Proposed add below functions:

func IsSorted[E cmp.Ordered](s Seq[E])

func IsSortedFunc[E any](s Seq[E], f func(E, E) int)

[Merovius]

If the IsSortedFunc is added, the comparator should be a three-way comparison, for the same reason that slices.IsSortedFunc and slices.SortStableFunc use it:

func IsSortedFunc[E any](s iter.Seq[E], cmp func(E, E) int)

for similar reasons as with #67458 I'm not sure about the utility of these: Checking if a sequence is sorted while also throwing it away seems kind of rarely useful.

[leaxoy]

A typo, thanks for point out.

[seankhliao]

xiter was withdrawn #61898