This removes a strange internal match mode _Skip_zero_length. Its only purpose was to shift the start of the pattern search one position later, but to set prefix().first in the match_results object as if there were no such shift. This match mode was only used to implement the increment operator of regex_iterator.

Moreover, _Regex_search2 has an additional parameter _Org which is only used to set the member _Org in the match_results object.

This PR removes this internal match mode _Skip_zero_length and replaces it in regex_iterator by using another internal match mode, _Match_not_null. This sometimes reduces the number of constructed matcher objects in the zero-length match case, which can improve performance by avoiding some unnecessary allocations. But the fusion of two regex_search calls means that the implementation deviates a bit further from the specification in [re.regiter.incr]: This necessitates some additional logic to emulate the match_prev_avail update to the match flags, because the original spec allows the caller to differentiate in some cases whether the first or the second call to regex_search() succeeded when incrementing from an initial zero-length match. The new tests make sure that this alternate logic behaves like the spec. (We do not have to emulate this logic strictly if the regex search fails, because the value of the match flags member is irrelevant for the end-of-sequence iterator.)

Moreover, this PR moves the update of the match_results field _Org outside _Regex_search2/3. Instead, the callers have to set this member correctly now. This allows the removal of the _Org parameter from _Regex_search2/3. Note that the callers were already responsible for choosing the value of _Org; they just didn't assign it themselves, but instead passed it to _Regex_search which performed the assignment (and did nothing else with this argument).

Drive-by change: _STD-qualify a few more calls.

Drive-by bugfix: I think regex_iterator::operator++() failed to call _Adopt(nullptr) in debug mode in one case when no more match was found.