* 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.

To keep the compilation firewall effect, the implementations
are hidden behind a PIMPL. In this case it is probably not
worth it, but we can inline it later if needed.

Also split out helpers for testing matcher ranges (types whose
begin/end/empty/etc require ADL lookup, types whose iteration
uses iterator + sentinel pair, etc) into their own file.