1. 'dim' is used for variables and affine expressions and constraints.
2. 'argdim' is used for objects with one particular argument of varying
   dimension. Bound terms of fixed dimension are not counted.
3. 'diag' is used instead of 'argdim' for objects defined on square
   matrices.

Furthermore, AffineConstraint.Subtype now has an 'eq' field for
distinguishing equalities and inequalities. (Needed for !85.)