patch 8.2.0559: clearing a struct is verbose

Problem:    Clearing a struct is verbose.
Solution:   Define and use CLEAR_FIELD() and CLEAR_POINTER().
diff --git a/src/ops.c b/src/ops.c
index 5f7cc60..5e9de4d 100644
--- a/src/ops.c
+++ b/src/ops.c
@@ -3412,7 +3412,7 @@
     void
 clear_oparg(oparg_T *oap)
 {
-    vim_memset(oap, 0, sizeof(oparg_T));
+    CLEAR_POINTER(oap);
 }
 
 /*