Clarifications in the README.

Fixed natvis and increased version to 1.5.2.

The sentinel for 64 bit pointers got changed in 402ac93. I forgot to adapt the sentinel in natvis.

Increased version to 1.5.1.

When I increased the version to 1.5.0 in c42bc18, I unfortunately forgot to increase the version in the CMakeLists.txt.
This commit fixes it, and increases the patch version to 1 to distinguish it from the faulty 1.5.0.

Fixed msvc tests in release.

The test_Exceptions() caused the link time optimization to complain about unreachable code. The "#pragma warning(disable : 4702)" at the top of ConstructionTests.cpp did not work because tiny/optional.h got included before indirectly in TestUtilities.h.
I think it is better in general to keep tiny/optional.h out of TestUtilities.h, so removed the dependency again.

Made versions with same major and minor but different patch versions …

…compatible to each other + incremented version to 1.4.0.

I.e. we guarantee compatibility between versions of the library with different patch versions if the major and minor versions are the same. This allows to make releases with just e.g. documentation or test updates, where the patch version gets incremented and now no longer leads to a breaking change.

Amended Natvis to account for the new inline namespace.

Unfortunately, Natvis needs the inline namespace to resolve the types.
This also means that both the version number and the configuration gets
hardcoded in Natvis. I think there is no way around this. (We have the
same problem with e.g. the abseil library.)
So at least explain it in the Natvis and the readme.

Updated readme explaining how to disable the UB tricks.

Fixed natvis when std::nullopt does not appear in any translation unit.

If std::nullopt is not used in any .cpp file, Visual Studio does not
seem to know about it. Natvis then throws errors.
To work around this, simply use a string.

Updated github workflows to use more recent machines and compilers.

Updated comment that gcc 13.2 still does not implement CWG 2445.