py: Make long ints hashable.

Addresses issue #765.
diff --git a/py/objint.c b/py/objint.c
index bca32d1..e351c3f 100644
--- a/py/objint.c
+++ b/py/objint.c
@@ -215,6 +215,10 @@
 
 #if MICROPY_LONGINT_IMPL == MICROPY_LONGINT_IMPL_NONE
 
+mp_int_t mp_obj_int_hash(mp_obj_t self_in) {
+    return MP_OBJ_SMALL_INT_VALUE(self_in);
+}
+
 bool mp_obj_int_is_positive(mp_obj_t self_in) {
     return mp_obj_get_int(self_in) >= 0;
 }