Improve MultiIndex pk deserialization

[maurolacy]

Signal the primary key deserialization type of MultiIndex using a PK trait or similar, like in UniqueIndex. (done in #568)

In passing, rename K to IK (Index Key) for clarity in both indexes. (done in #569)

If possible, make those PK traits automatic / inherited from the IndexedMap specification. That means, better encapsulation / coupling between IndexedMap and the *Index impls.

[maurolacy]

Breaking change, for 0.11, or 0.12.

[ethanfrey]

Is this done? @maurolacy

If so, let's close. If not, can you improve the description so we can follow up on it?

[maurolacy]

This is "closed" by the index_trait procedural macro.

Implementing the third point in the description would require a significant refactor of IndexedMap and associated indexes.

So, I would say, let's close it as solved through a workaround.