Store ValueExprNodes instead of jrd_nod in the metadata cache.
Make RecordSourceNode child of ExprNode as they share most operations.
Get rid of the JRD visitors in favor of direct calls.
Convert assignments statement lists created inside expressions to separate source and targets ValuesExprNodes.
2) Created ExprNode::expressionEqual method to replace a new big and ugly switch.
3) Make boolean referencers use BoolExprNode instead of jrd_nod, except nod_validate for now. This includes BinaryBoolNode and NotBoolNode working without jrd_nod references.
4) Rework in the impure allocation code (replace "csb_impure += ..." by CMP_impure calls).