Rewrite `can_derive_copy[_in_array]` as either a graph traversal or fix-point analysis

[fitzgen]

See #536 for details.

[fitzgen]

Note that in #767, we found it easier to compute the set of things that cannot derive Debug rather than the set of things that can.

[photoszzt]

I think this may involve two sets instead of one set. Named would not be the cannot_derive_copy set but it would be in the cannot_derive_copy_in_array set. Can the monotone framework return two sets instead of one?

[fitzgen]

Can the monotone framework return two sets instead of one?

Yep!

[photoszzt]

@highfive assign me

[photoszzt]

Looks like highfive doesn't work. I'm working on this.