Change MultiIndex index function signature to include the pk

[maurolacy]

It would be good that the MultiIndex index function signature, instead of being fn(&T) -> IK, was fn(&PK, &T) -> IK instead.

That way, the pk (or parts of it) can be used for indexing. And so the index keys don't have / need to come from the values alone.

[ewoolsey]

Any progress being made on this?

[maurolacy]

Shouldn't be too hard to do this change. Delayed mostly because it's a breaking change. Will give it a try.