Add user-defined literal support for string_constant

[cmazakas]

We need a way of supporting a syntax like:

std2::string_constant<char> str = "hello, world!"sc;

Currently, this doesn't seem possible as there's no constructor for string_constant from a char pointer, such as here:

#feature on safety
#include <std2.h>

using namespace std2;

inline auto operator""sv2(std2::string_constant<char> sc) noexcept safe -> string_view/static {
  return string_view(sc);
}

int main() {
  string_view sv = "Hello"sv2;
}