Add a folding version of the Cat ValueExpression

[jvdb]

As discussed in #229, long chains of concatenated data are a challenge for the 7.0.0 version of ConcatenatedValueSource. In order for a list-based implementation to work without too much instanceof magic, it makes sense to have a version of Cat that simply takes a single ValueExpression operand and then concatenate all the results it evaluates to. This is essentially the same as passing that operand to the Fold ValueExpression and providing the Cat ValueExpression as reducer.

This could be generalized to all BinaryValueExpressions, see #231 for a discussion of that.