Rule: unification used for variable assignment

[anderseknert]

Avoid

x = 5

Prefer

x := 5

Motivation: while there are a few valid cases for unification, the unification operator (=) should not be used for simple assignment. TBD — more comprehensive explanation.

[anderseknert]

The tricky thing in both this issue and #4 will be to determine whether unification really is intended — i.e. other constructs are less effective / elegant. For starters, I don't think we'll need to be too comprehensive in this check but just try and catch the simpler cases of one var/value on either side.

[anderseknert]

There is however a great number of cases where the := operator should be used unconditionally, and these would be easy to write checks for:

default allow = false 

f(x) = x

foo[x] = y { }

EDIT: And we do that now.

[anderseknert]

Closing in favor of #4, which will be remodeled to be "Unnecessary unification".