Improve representation of Scheme vectors

[jevdplas]

Similar to the new representation of cons-cells, the representation of vectors in Scheme should be modified. Currently, in ModularSchemeLattice.scala, a vector and all its elements are represented as a single value in the store with the following representation:

case class Vec(size: I, elements: Map[I, L], init: L) extends Value

Ideally, the sub-values of a vector should each be stored at their own address, similar to how a separate address is allocated for the car and the cdr of each cons-cell. However, unlike with cons-cells, this modification will be non-trivial, because:

• a vector can have an arbitrary number of elements
• both the size and indices of a vector can use any arbitrary abstract integer domain

[noahvanes]

The proposed solution is no longer in line with the current representation of reference types.