Make Int inherit from Ord.

Previously Int inherited from PartialOrd (via Primitive)
but not Ord.  But integers have a total order, so
inheriting from Ord is appropriate. Fixes #18776.
auto merge of #18795 : haberman/rust/master, r=cmr

Previously Int inherited from PartialOrd (via Primitive)
but not Ord.  But integers have a total order, so
inheriting from Ord is appropriate. Fixes #18776.