Define what expressions can be put in a colour declaration

[falkecarlsen]

Currently only integer_literals are allowed in a state colour declaration. Adding support for expressions, and more importantly, restricting these needs to be done.

[jenrik]

I think it would be best if we limited expressions in the colors to things that can be computed at compile time. Loop and functions should probably be forbidden since we can't always determine if they will terminate. Functions could enter an infinite loop through recursion.

We could check for the presence of certain nodes in the color expression by running a custom sanity checking visitor on the color expression subtree.

[jenrik]

#30 discusses the same issue, and has suggestions on what should be allowed