patch 8.2.0928: many type casts are used for vim_strnsave()

Problem:    Many type casts are used for vim_strnsave().
Solution:   Make the length argument size_t instead of int. (Ken Takata,
            closes #5633)  Remove some type casts.
diff --git a/src/digraph.c b/src/digraph.c
index dd5920a..d4175d5 100644
--- a/src/digraph.c
+++ b/src/digraph.c
@@ -2387,10 +2387,10 @@
 	{
 	    kp = (kmap_T *)curbuf->b_kmap_ga.ga_data + curbuf->b_kmap_ga.ga_len;
 	    s = skiptowhite(p);
-	    kp->from = vim_strnsave(p, (int)(s - p));
+	    kp->from = vim_strnsave(p, s - p);
 	    p = skipwhite(s);
 	    s = skiptowhite(p);
-	    kp->to = vim_strnsave(p, (int)(s - p));
+	    kp->to = vim_strnsave(p, s - p);
 
 	    if (kp->from == NULL || kp->to == NULL
 		    || STRLEN(kp->from) + STRLEN(kp->to) >= KMAP_LLEN