Add variable ivar to RBVariableNode

RBVariableNode holds onto the semantic vars using a property.

Now that we seldomly yes the AST without doing name analysis, this is not really good: we pay for the dictionary both in space and speed.

This PR adds the ivar (as it can not be done form the image) but does not yet hook it up.

src/AST-Core/RBVariableNode.class.st

@@ -15,6 +15,7 @@ Class {
 	#superclass : #RBValueNode,
 	#instVars : [
 		'name',
+		'variable',
 		'start'
 	],
 	#category : #'AST-Core-Nodes'