VPDomain/array equality domain: arrays as values

[alexandernutz]

so far we had a strict separation between arrays and values/nodes/elements

We had a special case for multidimensional arrays.

We want to support the more general case where arrays can be elements, too.

For example if a is a two-dimensional array, a[i] has both aspects: it is a value in a, but it is also a one-dimensional array.

steps on the way:

• should obsolete any occurence of the MultiDimensionalStore wrapper class
• adapt the TransFormula to EqConstraint conversion
• rework the congruence closure class to remove the strict separation between function and element
• implement the more general propagation rules