PERF: (GH3733) where block splitting now done in at most 2 blocks rather than

      item-by-item; add vb_suite tests for masking
BUG: _interleave now returns an lcd of int64 or 2 x the intsize if

     uints are seen
Merge pull request #3736 from jreback/mask_slow

PERF: speed up where operations when splitting blocks (GH3733)