Summary:
Move Benchmark.cpp out of the main libfolly library and into its own
libfollybenchmark library.

This more closely follows the build artifacts produced by the autoconf build,
and will help avoid breaking downstream projects that depend on folly and
expect to find libfollybenchmark.  The code in Benchmark.cpp is also normally
only needed by test programs.

Reviewed By: yfeldblum

Differential Revision: D6793538

fbshipit-source-id: fd273c11fc9f199a159b9e5d14badaf805480db0