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
Clarify encoding kwarg on to_csv
DOC: Clean up Stata documentation

Clean up and simplification of Stata documentation.

[skip ci]
Merge pull request #8753 from immerrr/refactor-slice-locs

API: allow negative steps for label-based indexing
Merge pull request #8858 from bashtage/stata-doc-fixup

DOC: Clean up Stata documentation
Merge pull request #8857 from graingert/patch-1

Clarify encoding kwarg on to_csv
BUG: type change breaks BlockManager integrity
Merge pull request #8853 from behzadnouri/blk-mgr

BUG: type change breaks BlockManager integrity
index into multi-index past the lexsort depth
BUG: missing nose import for skip test

nose.SkipTest used but nose not imported.

Import nose at top level and use that throughout.
Merge pull request #8867 from matthew-brett/missing-nose-import

BUG: missing nose import for skip test
Merge branch 'mi-key-loc' of https://github.com/behzadnouri/pandas into behzadnouri-mi-key-loc
TST: provide a clear argument to assert_produces_warning to guarantee that we are clearing
     specific warnings before trying to catch them (in case they have already happened)
     requires the user to specificy the exact class where they are coming
BUG CSV: fix problem with trailing whitespace in skipped rows, issues 8661, 8679
ENH CSV: Reduce memory usage when skiprows is an integer in read_csv, issue 8681