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/gui_mac.c b/src/gui_mac.c
index 875b3ec..b37d3f5 100644
--- a/src/gui_mac.c
+++ b/src/gui_mac.c
@@ -5451,7 +5451,7 @@
     button = 0;
 
     // initialize the hotkey mapping
-    vim_memset(hotKeys, 0, sizeof(hotKeys));
+    CLEAR_FIELD(hotKeys);
 
     for (;*buttonChar != 0;)
     {