BUG: fix unaligned access of new indexing

Requires 16 bytes alignment from string and flexible dtypes, as
processing functions might access them on the itemsize which can be
larger than 1 byte (e.g. 8 bytes strings)
16 byte the largest alignment required for all numpy copy loops.
Closes gh-4314
Merge pull request #4544 from juliantaylor/unaligned-index

BUG: fix unaligned access of new indexing