"manually" implement PartialOrd when we cannot derive it

[fitzgen]

Depends on #882.

We should be conservative and only implement this when the reason we cannot derive is because we're hitting the array length derive limit and similar easy cases. We can't know all the corner cases for types we have no knowledge of, and we shouldn't try.

[pepyakin]

@highfive: assign me

[highfive]

Hey @pepyakin! Thanks for your interest in working on this issue. It's now assigned to you!

[pepyakin]

@highfive: unassign me
Realized that this depends on #879