* Successive executions of the same `GENERATE` macro (e.g. because
of a for loop) no longer lead to multiple nested generators.
* The same line can now contain multiple `GENERATE` macros without
issues.

Fixes #1913

In the future, we will also want to introduce our own
`uniform_int_distribution` and `uniform_real_distribution` to get
repeatable test runs across different platforms.

This way it is explicit when there is a `StringRef` -> `std::string`
conversion and makes it easier to look for allocations that could
be avoided.

Doing this has already removed one allocation per registered test
case, as there was a completely pointless `StringRef` -> `std::string`
conversion when parsing tags of a test case.

* 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

This adds UNSCOPED_INFO macro, creating a log message that is stored
until the end of next assertion or the end of test case, whichever comes
first. These messages are not scoped locally, unlike messages created by
INFO macro.

Fixes #1514