feat(order/ideal): add partial_order instance to order.ideal (#5909)
feat(order/ideal): add partial_order instance to order.ideal (#5909)

Add some instances for `order.ideal`, some of them conditional on
having extra structure on the carrier preorder `P`:

* In all cases, `ideal P` is a partial order.

* If `P` has a bottom element, so does `ideal P`.

* If `P` has a top element, so does `ideal P`.
  (Although this could be weekened to `P` being directed.)

Also, add some `@[ext]`, `@[simp]`, `@[trans]` lemmas.






leanprover-community/mathlib,leanprover-community/mathlib