benchmark

Add `BENCHMARK_TEMPLATE[12]_CAPTURE`, fusion of `BENCHMARK_CAPTURE` and `BENCHMARK_TEMPLATE` (#1747)

Test coverage isn't great, but not worse than the existing one.

You'd think `BENCHMARK_CAPTURE` would suffice,
but you can't pass `func<targs>` to it (due to the `<` and `>`),
and when passing `(func<targs>)` we get issues with brackets.
So i'm not sure if we can fully avoid this helper.

That being said, if there is only a single template argument,
`BENCHMARK_CAPTURE()` works fine if we avoid using function name.