refactor: use CLEAR_FIELD and CLEAR_POINTER macros (#19709)

vim-patch:8.2.0559: clearing a struct is verbose

Problem:    Clearing a struct is verbose.
Solution:   Define and use CLEAR_FIELD() and CLEAR_POINTER().
a80faa8930

src/nvim/lua/converter.c

@@ -59,7 +59,7 @@ static LuaTableProps nlua_traverse_table(lua_State *const lstate)
   size_t other_keys_num = 0;  // Number of keys that are not string, integral
                               // or type keys.
   LuaTableProps ret;
-  memset(&ret, 0, sizeof(ret));
+  CLEAR_FIELD(ret);
   if (!lua_checkstack(lstate, lua_gettop(lstate) + 3)) {
     semsg(_("E1502: Lua failed to grow stack to %i"), lua_gettop(lstate) + 2);
     ret.type = kObjectTypeNil;

src/nvim/lua/executor.c

@@ -447,7 +447,7 @@ static nlua_ref_state_t *nlua_new_ref_state(lua_State *lstate, bool is_thread)
   FUNC_ATTR_NONNULL_ALL
 {
   nlua_ref_state_t *ref_state = lua_newuserdata(lstate, sizeof(*ref_state));
-  memset(ref_state, 0, sizeof(*ref_state));
+  CLEAR_POINTER(ref_state);
   ref_state->nil_ref = LUA_NOREF;
   ref_state->empty_dict_ref = LUA_NOREF;
   if (!is_thread) {

src/nvim/lua/xdiff.c

@@ -269,9 +269,9 @@ int nlua_xdl_diff(lua_State *lstate)
   xpparam_t params;
   xdemitcb_t ecb;
 
-  memset(&cfg, 0, sizeof(cfg));
-  memset(&params, 0, sizeof(params));
-  memset(&ecb, 0, sizeof(ecb));
+  CLEAR_FIELD(cfg);
+  CLEAR_FIELD(params);
+  CLEAR_FIELD(ecb);
 
   NluaXdiffMode mode = kNluaXdiffModeUnified;