Move number string parsing from the parser into the reducer.

core/src/main/kotlin/org/nwapw/abacus/tree/NumberNode.kt

@@ -10,14 +10,14 @@ import org.nwapw.abacus.number.NumberInterface
  *
  * @number the number value of this node.
  */
-data class NumberNode(val number: NumberInterface) : TreeNode() {
+data class NumberNode(val number: String) : TreeNode() {
 
     override fun <T : Any> reduce(reducer: Reducer<T>): T? {
         return reducer.reduceNode(this)
     }
 
     override fun toString(): String {
-        return number.toString()
+        return number
     }
 
 }