All the previous refactoring to make the assertion fast paths
smaller and faster also allows us to implement the fast paths
just with thread-local and atomic variables, without full mutexes.

However, the performance overhead of thread-safe assertions is
still significant for single threaded usage:

|  slowdown |  Debug  | Release |
|-----------|--------:|--------:|
| fast path |   1.04x |   1.43x |
| slow path |   1.16x |   1.22x |

Thus, we don't make the assertions thread-safe by default, and instead
provide a build-time configuration option that the users can set to get
thread-safe assertions.

This commit is functional, but it still needs some follow-up work:
 * We do not need full seq_cst increments for the atomic counters,
   and using weaker ones can be faster.
 * We brute-force updating the reporter-friendly totals from internal
   atomic counters by doing it everywhere. We should properly trace
   where this is needed instead.
 * Message macros (`INFO`, `UNSCOPED_INFO`, `CAPTURE`, etc) are not
   made thread safe in this commit, but they can be made thread safe
   in the future, by building on top of this work.
 * Add more tests, including with thread-sanitizer, and compiled
   examples to the repository. Right now, these changes have been
   compiled with tsan manually, but these tests are not added to CI.

Closes #2948

This helps the compiler see that the execution does not continue,
which in turn means that it is not an error if the user does not
return value from the function after FAIL/SKIP.

Closes #2941

Move `catch_test_run_info.hpp` from directory
`src/catch2/internal/` to be directly under `src/catch2/`.

Header `catch_test_run_info.hpp` defines `Catch::TestRunInfo` which is part of the public API, since it is required when overriding `Catch::EventListenerBase`.

Fixes issue caught by Clang Tidy misc-include-cleaner check.

Move `catch_case_sensitive.hpp` from directory
`src/catch2/internal/internal` to be directly under `src/catch2/`.

Header `catch_case_sensitive.hpp` defines `enum CaseSensitive` which is
part of the public API, since it is exposed by the string matcher
contracts.

Fixes issue caught by Clang Tidy misc-include-cleaner check.

* Utility for extended mult n x n bits -> 2n bits
* Utility to adapt output from URBG to target (unsigned) integral
  type
* Utility to reorder signed values into unsigned type while keeping
  the order.

Specifically we add
 * `gamma(a, b)`, which returns the magnitude of largest 1-ULP
   step in range [a, b].
 * `count_equidistant_float(a, b, distance)`, which returns the
   number of equi-distant floats in range [a, b].

Co-authored-by: Martin Hořeňovský <martin.horenovsky@gmail.com>