Make enum variants more consistent between exercises

[matthri]

Make sure the Point struct is already introduced in enums2 and can get reused in the following exercise.
I added an additional Resize variant since I think it is important to highlight the difference between the wrapped Point struct and an anonymous struct.

Additionally I changed the order of the listed variant types of the enums2 exercise to match the order of the variants in the code to make the connection a bit clearer.

closes #1545

[mo8it]

Thanks :D

I polished the exercises a bit and adjusted the solutions in 584164a.