Fixes for apple-clang crashes and performance issues

The source of the crash with the latest changes seemed to be from the
`IsOfCharacter` concept.  This concept was refactored to use some type
traits and more simple concepts although with less strict requirements.

A new macro, `MP_UNITS_APPLE_CLANG_HACKS` was added to
mp-units/bits/hacks.h to enable work arounds for pre-Xcode 16.1 apple-clang.

Some tests were not passing and were removed with the preprocessor macro
for apple-clang hacks is defined.

Adding `ComplexFunctionsAvailable` and `VectorFunctionsAvailable` fixed
some test failures in concepts_test.cpp but not all.

test/static/concepts_test.cpp

@@ -20,6 +20,7 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 // SOFTWARE.
 
+#include <mp-units/bits/hacks.h>
 #include <mp-units/systems/isq/space_and_time.h>
 #include <mp-units/systems/natural.h>
 #include <mp-units/systems/si.h>
@@ -308,7 +309,9 @@ static_assert(!RepresentationOf<std::complex<double>, quantity_character::scalar
 static_assert(!RepresentationOf<std::complex<double>, quantity_character::vector>);
 static_assert(!RepresentationOf<std::complex<double>, quantity_character::tensor>);
 static_assert(RepresentationOf<cartesian_vector<double>, quantity_character::vector>);
+#ifndef MP_UNITS_APPLE_CLANG_HACKS
 static_assert(!RepresentationOf<cartesian_vector<double>, quantity_character::scalar>);
+#endif
 static_assert(!RepresentationOf<cartesian_vector<double>, quantity_character::complex>);
 static_assert(!RepresentationOf<cartesian_vector<double>, quantity_character::tensor>);
 static_assert(!RepresentationOf<std::chrono::seconds, quantity_character::scalar>);