std::string_view initialization support in val.h

Currently `val` class has a few constructors for different basic types. It would be nice to have `std::string_view` type constructor too. The only difference with `const char*` is that `std::string_view` is not necessarily null-terminated, instead, it holds the length information.

I guess it's a very low priority feature request. But is this something acceptable if someone implements and opens a PR?