Added support for assignable function results.

Also:
- Added megabuf(index) as a first, temporary example
- Introduced a new Node class at the top of the hierarchy for the runtime.
- RValue and LValue are now interfaces
- Narrowed down some exception declarations
- Optimized the optimizer for functions

src/main/java/com/sk89q/worldedit/expression/Identifiable.java

@@ -41,8 +41,9 @@ public interface Identifiable {
      *
      * Invokables:
      * c - Constant
-     * f - Function
      * v - Variable
+     * f - Function
+     * l - LValueFunction
      * </pre>
      */
     public abstract char id();