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/builtin.rs

@@ -13,12 +13,13 @@ pub static PRINTLN_BUILTIN_FUN: Lazy<NativeFunRef> = Lazy::new(|| NativeFun::new
             .expect("no __str__ or __repr__ member")
     };
     let return_value = vm.call(to_string, vec![]);
-    let str_ref: &StrRef = std::any::Any::downcast_ref(&return_value).expect("to_string to return str value");
     {
-        read_obj!(let str_obj = str_ref);
+        read_obj!(let str_obj = return_value);
+        let str_obj: &Str = str_obj.as_any().downcast_ref().unwrap();
         println!("{}", str_obj.value());
     }
 
+    vm.push(NIL_NAME.sym_ref());
     Signal::Return
 }));
 
@@ -32,9 +33,9 @@ pub static PRINT_BUILTIN_FUN: Lazy<NativeFunRef> = Lazy::new(|| NativeFun::new_o
             .expect("no __str__ or __repr__ member")
     };
     let return_value = vm.call(to_string, vec![]);
-    let str_ref: &StrRef = std::any::Any::downcast_ref(&return_value).expect("to_string to return str value");
     {
-        read_obj!(let str_obj = str_ref);
+        read_obj!(let str_obj = return_value);
+        let str_obj: &Str = str_obj.as_any().downcast_ref().unwrap();
         print!("{}", str_obj.value());
     }