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

P3168 turned `std::optional` into a range type, so the partial specialization
of `StringMaker` for `std::optional<T>` conflicted with the partial
specialization for range types. Ideally we will fix this in the future
to support user-provided partial specializations for range-like types,
but for now we just disable the partial specialization for `std::optional<T>`
if P3168 is implemented.