patch 8.1.1807: more functions can be used as a method

Problem:    More functions can be used as a method.
Solution:   Add append(), appendbufline(), assert_equal(), etc.
            Also add the :eval command.

src/ex_eval.c

@@ -870,6 +870,18 @@ report_discard_pending(int pending, void *value)
 }
 
 
+/*
+ * ":eval".
+ */
+    void
+ex_eval(exarg_T *eap)
+{
+    typval_T	tv;
+
+    if (eval0(eap->arg, &tv, &eap->nextcmd, !eap->skip) == OK)
+	clear_tv(&tv);
+}
+
 /*
  * ":if".
  */