Fix generic bound of `str::SplitInclusive`'s `DoubleEndedIterator` impl by timve...

str::SplitInclusive's DoubleEndedIterator implementation currently uses a ReverseSearcher bound for the corresponding searcher. A DoubleEndedSearcher bound should have been used instead.

DoubleEndedIterator requires that repeated next_back calls produce the same items as repeated next calls, in opposite order. ReverseSearcher lets you search starting from the back of a string, but it makes no guarantees about how its matches correspond to the matches found by a forward search. DoubleEndedSearcher is a subtrait of ReverseSearcher and does require that the same matches are found in both directions.

This bug fix is a breaking change. Calling next_back on "a+++b".split_inclusive("++") is currently accepted with repeated calls producing "b" and "a+++", while forward iteration yields "a++" and "+b". Also see #100756 (comment) for more details.

I believe that this is the only iterator that uses this bound incorrectly — other related iterators such as str::Split do have a DoubleEndedSearcher bound for their DoubleEndedIterator implementation. And slice::SplitInclusive doesn't face this problem at all because it doesn't use patterns, only a predicate.

cc @SkiFire13