This replaces all use of std::optional's `opt.value()` with `*opt`
because macOS is great and the ghost of Steve Jobs says that actually
supporting std::optional's value() method is not for chumps before macOS
10.14. So don't use it because Apple is great.
Pretty much all of our use of it actually is done better with operator*
anyway (since operator* doesn't do a check that the optional has a
value).
Also replaced *most* of the `has_value()` calls with direct bool
context, except for one in the config section which looked really
confusing at a glance without a has_value().
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)