This is an initial pass at doing explicit value checks when handling
config parsing, as opposed to using a visiting pattern. The latter
made it difficult to check for conditions such as missing required
values, multiple values, etc.
It was also generally less readable (think declarative) which further
made it difficult to get a grasp for what our actual configuration file
requirements were.
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)
