The old implementation of String matchers worked by making a, potentially
lower-cased, copy of both the constructor arg, and the arg to `match` calls.

The new implementation does not make copies, and instead does per-character
lowering if needed. This is a win in **most** cases, with two exceptions:

1) Case-insensitive `Equals` between long strings. The old implementation
   is easy to autovectorize by the compiler, which wins over the extra copy
   with sufficiently long inputs.
2) Case-insensitive `ContainsSubstring` with inputs that trigger the worst
   case complexity of O(m*n) of the naive implementation.

The latter is fixable with some elbow grease by implementing a proper
string search algorithm, but the former is a cost we have to live with.

To make this all work, I had to remove the stringification cache
from matchers. In theory, this can cause performance penalty in
cases where single matcher instance is stringified multiple times,
but in practice this does not happen much, and the difference is
surprisingly small anyway, because the performance of stringification
is already horrible and full of allocating strings just to throw
them away.

The matcher combinators need P2738 from C++26 to be `constexpr`.

Closes #3091

The matcher combinators do not take ownership of the matchers
being combined, which can catch the users off-guard, when code like
this

```cpp
using Catch::Matchers::EndsWith;
using Catch::Matchers::ContainsSubstring;

auto combinedMatcher = EndsWith("as a service")
                       && ContainsSubstring("web scale");

REQUIRE_THAT( getSomeString(), combinedMatcher );
```

leads to use-after-free, as the `combinedMatcher` refers to matcher
temporaries that no longer exists. With this commit, users of Clang,
MSVC or other compiler that understands the `lifetimebound` attribute,
should get a warning.