Support Enums as config items

[jamiesanson]

We're not able to enforce type constraints on config providers themselves, but from an end-user perspective it'd be useful to be able to model config items as enums straight out of the box. By default the mapping would be to and from the enum "value", making use of Enum Generics.

An example of how this might look:

enum class ThemeAccent {
    LightBlue,
    BrightGreen
}

// in SubConfig implementation
val themeAccent by config<ThemeAccent>()

Implementation-wise, the delegate function can be selected via overload resolution. An implementation such as this should suffice:

inline fun <reified T: Enum<T>> ConfigRegistry.config(/*...*/): ConfigRegistrar<T> = ...