arrow

ARROW-3242: [C++] Make CpuInfo a singleton, use coarser-grained dispatch to SSE4 in Parquet dictionary encoding

Rather than having a bunch of static state in a header file (which is not a great pattern anyway), this makes `arrow::CpuInfo` a singleton. I added support for vector-level dispatch to SSE4 hashing in the Parquet dictionary encoder.

This solves ARROW-3241 for me

NB. This is precisely the kind of change that is now radically simpler after the monorepo merge

Author: Wes McKinney <wesm+git@apache.org>

Closes #2571 from wesm/ARROW-3242 and squashes the following commits:

cc66a4064 <Wes McKinney> Remove cruft from mid-refactor
c09aaea50 <Wes McKinney> Do not link librt on Apple platform
abacaf4b8 <Wes McKinney> Make CpuInfo a singleton, use coarser-grained dispatch to SSE4 in Parquet dictionary encoding