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
Merge pull request #6269 from jreback/perf2

PERF: micro optimizations