BUG: fix negative step support for label-based slices

INT: make Index.slice_locs step-aware

BUG: fix PeriodIndex.searchsorted to accept Periods

INT: refactor time-related indices to use step-aware slice_locs

INT: refactor MultiIndex to use step-aware slice_locs

INT: enable second/microsecond partial string slicing
Merge pull request #8753 from immerrr/refactor-slice-locs

API: allow negative steps for label-based indexing