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/obj/fun.rs

@@ -11,6 +11,61 @@ pub trait Fun {
     fn create_frame(&self, callee: ObjRef, vm: &Vm, _args: Vec<ObjRef>) -> Frame;
 }
 
+pub type MethodRef = ObjRef<Method>;
+
+#[derive(Scan, Debug)]
+pub struct Method {
+    vtable: Vtable,
+    attrs: Attrs,
+}
+
+impl Method {
+    pub fn new_obj(owner: ObjRef, function: ObjRef) -> MethodRef {
+        self_referring_obj! {
+            Self {
+                vtable: Default::default(),
+                attrs: attrs! {
+                    SELF_MEMBER_NAME.sym => owner,
+                    FUNC_MEMBER_NAME.sym => function,
+                }
+            },
+            vtable: |obj_ref: ObjRef| vtable! {
+                CALL_MEMBER_NAME.sym => obj_ref.clone(),
+            }
+        }
+    }
+}
+
+impl Obj for Method {
+    fn vtable(&self) -> &Vtable {
+        &self.vtable
+    }
+
+    fn attrs(&self) -> &Attrs {
+        &self.attrs
+    }
+
+    fn attrs_mut(&mut self) -> Option<&mut Attrs> {
+        Some(&mut self.attrs)
+    }
+
+    fn as_any(&self) -> &dyn std::any::Any { self }
+
+    fn as_fun(&self) -> Option<&dyn Fun> { Some(self) }
+}
+
+impl Fun for Method {
+    fn create_frame(&self, _callee: ObjRef, vm: &Vm, mut args: Vec<ObjRef>) -> Frame {
+        // insert self argument
+        args.insert(0, self.get_attr(SELF_MEMBER_NAME.sym).unwrap());
+        // get function and use its create_frame method with the updated args
+        let fun_ref = self.get_attr(FUNC_MEMBER_NAME.sym).unwrap();
+        read_obj!(let fun_obj = fun_ref);
+        let fun = fun_obj.as_fun().unwrap();
+        fun.create_frame(fun_ref.clone(), vm, args)
+    }
+}
+
 //
 // struct UserFun
 //