string_view was implicitly convertible to std::string, but
std::string_view is only explicitly convertible. This makes the
`operator std::string` explicit to be more compatible, and re-adds a
bunch of explicit string casts to the code where needed.
(This also fixes the build if changing the standard to c++17)
Pre-C++17 char_traits::compare isn't constexpr so we can't constexpr the
find/rfind methods that use it.
begin() etc, however, can be constexpr (and need to be for some of the
other constexpr methods here that use them).
