[JExtract/JNI] Support enum cases

[colemancda]

Enum cases should be handled in the same way as static members are, only with their return value being Self.

[colemancda]

@madsodgaard Seems like a low hanging fruit, should reuse a lot of the code that static members uses right? If you don't have bandwidth now I was thinking of giving it a try tomorrow.

[madsodgaard]

Simple enums should be easily achievable yea. Things probably become a bit more complicated for raw representable, enum methods and associated values.

But I think we can start of by just importing simple enums. I will probably take a look at it in the next week or so, but you are also welcome to work on it, if you want to!:)