Ensure Parameter re-validates default if slot changes

[philippjfr]

This PR adds validation when overriding slot values on a Parameter in a subclass. It does this by checking if a superclass defines a slot value and if that slot value is overridden by the subclass. It attempts to minimize the number of actual lookups by aborting early if we already know that re-validation has to happen and by skipping checks on various slot values which do not affect validation.

Fixes #714

[philippjfr]

I had to disable re-validation of None values because the automatic appending of an unknown value on Selector opens a whole rabbit hole in regard to the validation.

[jbednar]

I'm having a good bit of trouble following the logic here, so hopefully we can work through it together. I think I probably wrote some of the confusing bits (e.g. what's "new" about what I think I was the one to call "new_param"?), but as the code has gotten more complex I'm now lost. The basic idea seems sound, i.e. to see if any slots have changed and thus whether we need to re-validate the default value.

[philippjfr]

@maximlt Can you either merge or ask for further clarification? You are release manager :)

[maximlt]

I was waiting for #826 to be merged! Now it's done I'll rebase this and merge if it goes well.