Enums: Make EnumDefinition use EnumValues instead of Strings as children.

This allows us to store the index of the enum value along the name. The index can be used to compare two enum values in assembler.

Later on this might be used to enable users of KLang to set arbitrary values as the index of an enum value.

src/main/java/de/hsrm/compiler/Klang/nodes/EnumValue.java

@@ -0,0 +1,18 @@
+package de.hsrm.compiler.Klang.nodes;
+
+import de.hsrm.compiler.Klang.visitors.Visitor;
+
+public class EnumValue  extends Node {
+    public String value;
+    public int index;
+
+    public EnumValue(String value, int index) {
+        this.value = value;
+        this.index = index;
+    }
+
+    @Override
+    public <R> R welcome(Visitor<R> v) {
+        return v.visit(this);
+    }
+}