More specifically, made the actual implementation of string-like
type handling take argument as `Catch::StringRef`, instead of
taking `std::string const&`.
This means that string-like types that are not `std::string` no
longer need to pay for an extra construction of `std::string`
(including the potential allocation), before they can be stringified.
The actual string stringification routine is now also better about
reserving sufficient space.
Martin Hořeňovský
9137e591fa