CLN: More work on converting assert_'s to specialized forms

Work on #6175. Tests still pass locally. A work-in-progress.
Merge pull request #6259 from jreback/xs_bug

BUG: bug in xs for a Series with a multiindex (GH6258)
CLN: Work on converting assert_'s to specialized forms, in pandas/tests

Work on #6175, in pandas/tests. Tests still pass locally. A work-in-progress, but--with #6261--removes many/most cases of assert_(x == y) in this directory
Merge pull request #6261 from bwignall/assert_6175_4

CLN: More work on converting assert_'s to specialized forms
Merge pull request #6262 from bwignall/assert_6175_5

CLN: Work on converting assert_'s to specialized forms, in pandas/tests
PERF: enhance tseries getitem indexing

PERF: micro optimzations in inference

API/CLN: core.internals.apply now only takes kwargs; makes code a bit simpler

CLN: wrap core/common/isnull into more direct Block.isnull

PERF: fast path series block operations