Support if! versus if

[DavePearce]

This presents some challenges because if performs some kind of inference. For example, consider this:

(defcolumns X Y A)
(defconstraints c1 () (if (eq! X Y) A))
(defconstraints c () (if (eq X Y) A))

Both constraints compile down to the same underlying form. One solution to this issue would be to remove use of if altogether from the constraints. That way, we can reinterpret its meaning as we see fit.

[DavePearce]

Another good example:

(defcolumns (X :binary@loob) (Y :binary@bool) A)
(defconstraints c1 () (if X A))
(defconstraints c2 () (if Y A))

These generate different constraints. The first is: if X == 0 then A whilst the second is if Y != 0 then A.

[DavePearce]

Fixed as part of #393