Support automatic value assignment to Enum members

[czgdp1807]

No description provided.

[certik]

Python does not have the best syntax here, but since this is what they do, we'll do it too for now. We can later fix this to something like:

@enum
class Color:
    RED: i32
    GREEN: i32
    BLUE: i32 = 7
    YELLOW: i32
    WHITE: i32
    PINK: i32 = 15
    GREY: i32

which seems nicer.

[certik]

I think that this is fine.