No default order for range types

use import mach.int.Int63
let rec foo (x: int63): int63
  variant { x }
= foo (x - 1)

File "foo.mlw", line 4, characters 2-13:
no default order for o

It would be good if range types were usable as variants without having to use of_int. In fact, the ordering relation is much simpler than in the int case, since we do not even have to choose an arbitrary lower bound.

Note also that the error message is rather poor since it mentions an automatically generated binder.