* Split out BenchmarkInfo and BenchmarkStats to their own header
* Outline BenchmarkStats<> declaration to separate header
* Split out TestRunInfo into its own header
These changes let us remove the large `interfaces_reporter.hpp`
include from `benchmark.hpp`, and replace it with
`interfaces_capture.hpp` in `run_context.hpp`.
I also cleaned out `interfaces_repoter.hpp` from reporter headers
that depend on `reporter_common_base.hpp`. This will not change
anything in the actual inclusion set, but makes it logically
more consistent.