This provides nice speedup when writing strings that don't need
escaping, at about 6% in Debug build and ~40% in Release build.
If the strings do need escaping the speedup is much smaller, at
~1% in Debug and ~6% in Release build, as the cost of actually
escaping the strings dwarves the cost of checking.

In practice, the majority of values the JSON writer handles are
strings, but this makes the code more obvious and can still provide
a small speed-up.

As the types and values sent into the writer are determined by Catch2,
I do not expect to actually need this support, but it is better to have
it and not be surprised in the future.

Closes #3176

The forward iteration logic already bounds-check for m_it != m_string->end(),
do the same for the backward iteration. The issue with the assert is
that the assert() might not be compiled in, and it is happening after
the dereference, so it was too late.

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