Add Method type for objects

* Methods are wrappers created with an owner and a function, which
  passes the owner as the first argument when the function is called.
* Fix a small bug in the VM where the pc was being set at the wrong
  time

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/vm/mod.rs

@@ -149,9 +149,11 @@ impl<'c> Vm<'c> {
                 let frame = callee_obj.as_fun()
                     .expect("callable function")
                     .create_frame(callee.clone(), self, args);
+                // Jump to the first address of the new function call if it's a user function
+                if let Frame::User(_) = &frame {
+                    self.set_pc(0);
+                }
                 self.frames_mut().push(frame);
-                // Jump to the first address of the new function call
-                self.set_pc(0);
             }
             Signal::Return => {
                 // 1. pop return value