Fix parameter types for rb_ivar_foreach() callbacks

For this public API, the callback is declared to take
`(ID, VALUE, st_data_t)`, but it so happens that using
`(st_data_t, st_data_t, st_data_t)` also
type checks, because the underlying type is identical.
Use it as declared and get rid of some casts.

error.c

@@ -3248,9 +3248,9 @@ exception_dumper(VALUE exc)
 }
 
 static int
-ivar_copy_i(st_data_t key, st_data_t val, st_data_t exc)
+ivar_copy_i(ID key, VALUE val, st_data_t exc)
 {
-    rb_ivar_set((VALUE) exc, (ID) key, (VALUE) val);
+    rb_ivar_set((VALUE)exc, key, val);
     return ST_CONTINUE;
 }