Also simplify some variables with auto deduction.

* Units from <ratio> are no longer redeclared in our own namespace
* The default clock is `steady_clock`, not `high_resolution_clock`,
because, as HH says "high_resolution_clock is useless. If you want
measure the passing of time, use steady_clock. If you want user
friendly time, use system_clock".
* Benchmarking support is opt-in, not opt-out, to avoid the large
(~10%) compile time penalty.
* Benchmarking-related options in CLI are always present, to decrease
the amount of code that is only compiled conditionally and making
the whole shebang more maintainble.

Changes done to Nonius:
* Moved things into "Catch::Benchmark" namespace
* Benchmarks were integrated with `TEST_CASE`/`SECTION`/`GENERATE` macros
* Removed Nonius's parameters for benchmarks, Generators should be used instead
* Added relevant methods to the reporter interface (default-implemented, to avoid
breaking existing 3rd party reporters)
* Async processing is guarded with `_REENTRANT` macro for GCC/Clang, used by default
on MSVC
* Added a macro `CATCH_CONFIG_DISABLE_BENCHMARKING` that removes all traces of
benchmarking from Catch

The `properties` can only go under the `testsuite` tag, not under
the `testsuites` tag.

Fixes #1598

Closes #1542

Some compilers don't export C compatibility functions in `::`. Using
`std::sprintf` should work for those (and all other compilers).

Reference: https://en.cppreference.com/w/cpp/header#C_compatibility_headers

According to TAP protocol version 13, the comments after the test name
only may contain Directives - # TODO or # SKIP. We should put
the comment (aka suite name) on a separate line before the test.

See http://testanything.org/tap-version-13-specification.html#directives

Fixes #1520