patch 8.1.1384: using "int" for alloc() often results in compiler warnings

Problem:    Using "int" for alloc() often results in compiler warnings.
Solution:   Use "size_t" and remove type casts.  Remove alloc_check(), Vim
            only works with 32 bit ints anyway.

src/digraph.c

@@ -2317,7 +2317,7 @@ keymap_init(void)
 	/* Source the keymap file.  It will contain a ":loadkeymap" command
 	 * which will call ex_loadkeymap() below. */
 	buflen = STRLEN(curbuf->b_p_keymap) + STRLEN(p_enc) + 14;
-	buf = alloc((unsigned)buflen);
+	buf = alloc(buflen);
 	if (buf == NULL)
 	    return e_outofmem;