common: tal_dup_talarr() helper.

This is a common thing to do, so create a macro.

Unfortunately, it still needs the type arg, because the paramter may
be const, and the return cannot be, and C doesn't have a general
"(-const)" cast.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

common/utils.c

@@ -141,3 +141,10 @@ void tal_arr_remove_(void *p, size_t elemsize, size_t n)
 	    len - (elemsize * (n+1)));
     tal_resize((char **)p, len - elemsize);
 }
+
+void *tal_dup_talarr_(const tal_t *ctx, const tal_t *src, const char *label)
+{
+	if (!src)
+		return NULL;
+	return tal_dup_(ctx, src, 1, tal_bytelen(src), 0, label);
+}