move Histogram.h to stats/

move Histogram.h to stats/

Summary:
Move Histogram.h into the stats/ subdirectory, along side the
BucketedTimeSeries code.  Eventually I plan to land more of our stats
code in this subdirectory too.

This also renames Histogram-inl.h to Histogram-defs.h, and no longer
includes it by default from Histogram.h.  Instead, this adds a new
stats/Instantiations.cpp file which explicitly instantiates
Histogram<int64_t> and BucketedTimeSeries<int64_t>.  Most callers use
these instantiations, and they now no longer need to include
Histogram-defs.h.  Only callers that need other instantiations need to
include Histogram-defs.h.  This will speed up build times.

Test Plan:
Ran "arc unit" to build all projects depending on folly, and verified
they all still built and passed tests.

Reviewed By: ldbrandy@fb.com

FB internal diff: D761377