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/charset.c b/src/charset.c
index aa843d1..7415c24 100644
--- a/src/charset.c
+++ b/src/charset.c
@@ -127,7 +127,7 @@
     /*
      * Init word char flags all to FALSE
      */
-    vim_memset(buf->b_chartab, 0, (size_t)32);
+    CLEAR_FIELD(buf->b_chartab);
     if (enc_dbcs != 0)
 	for (c = 0; c < 256; ++c)
 	{