allow enum casting not only enum value casting

[regnerisch]

This PR allows to cast enums, not just enum values.

[aidan-casey]

Thanks, @regnerisch! I think this is a good approach even for backed enums as it will now allow the value to be an enum instance. 👍🏼

[regnerisch]

That's the idea. I was a little irritated I have to do something like $data = MyData(origin: MyEnum::SOMETHING->value) to use the enum caster, so I created this PR :)