Faster hasing for `cvc5::String` (#7742)

Faster hasing for `cvc5::String` (#7742)

Previously, our hashing algorithm for cvc5::String would create an
std::string representation of the string and then call
std::hash<std::string> on the result. Creating the temporary
std::string is quite expensive and can be avoided by computing the
hash directly on the elements of the vector of characters in
cvc5::String.

This change improves our solving time on an industrial benchmark from
about 59s to 55s.