`ScopedEnum` wishlist

Issue to track more stuff I'd like to see from the `ScopedEnum`:

- [ ] Eliminate the need to register enums with a package-level `const ENUMS` for the purposes of `get_enum_value`
- [ ] Eliminate the need to manually define `Base.convert(::Type{MyEnum}, val::AbstractString) = get_enum_value(::MyEnum, val)`