Add support for string_view, adjust order of pretty_print functions

[Harry-Chen]

This PR mainly:

• Add specialization for pretty printing of std::string_view, avoiding it to be recognized as a container type and printed as something like {'t', 'e', 's', 't'}.
• Adjust the order or pretty print functions so that the specializations for sum types such as std::variant and container types comes last. Otherwise calling pretty_print to something like std::pair<std::string_view, std::string_view> still gives output like {{'t', 'e', 's', 't'}, {'t', 'e', 's', 't'}}, maybe due to the template resolution order of C++ (which I am not very familiar with). Change the order could solve this.

[sharkdp]

This looks great - thank you very much!

Adjust the order or pretty print functions so that the specializations for sum types such as std::variant and container types comes last

:+1: