Make JUCE string classes std::string_view compatible if C++17 is used

While using more and more C++17, I’d find it extremely handy if juce::String, juce::StringRef and juce::Identifier would offer explicit or implicit std::string_view conversion functions/operators. Would you consider adding such member functions, wrapped into JUCE_CXX17_IS_AVAILABLE guards? Possibly, StringRef could just become a std:: string_view wrapper in the long run?

Bumping, as I just ran into the need for something like that too.