MultiIndex primary key spec removal

[maurolacy]

Closes #533.

Builds on top of #568. Makes it simpler to handle MultiIndex by removing the pk from the index key specification.

Solved it by collapsing the index key along with an extra element (the raw pk). This makes it possible to keep the old format in the store, as the new (internally generated) index key would be exactly the same as the one corresponding to the previous key spec.

Overall, I'm pretty happy with this implementation. It has some tricks, like shifting the definition of what is a prefix and a sub-prefix. So that prefixes can still be specified over the full index key (which doesn't include the primary key in its spec anymore). Despite (or because of) that, I think the end result is both clear and ergonomic.

This PR also renames K to IK for clarity. So, it also closes (the second part of) #531.

TODO:

• Update related documentation / comments. (done)

[ueco-jb]

lgtm, mostly nitpicking comments regarding comments.

[ueco-jb]

This K -> IK change is quite sudden. I understand that K stands for Key, but is IK an IndexKey?
I think it would be good to have some reference to it... I doubt it will be clear to people diving into this code from outside.

[maurolacy]

It is an index key, yes. It's a way to differentiate it from both a PK (primary key), and from a generic K that could stand for any of them.

[ethanfrey]

I don't have the time to dig into this, which it deserves.

If you and Jake agree, feel free to merge.
I feel this is getting to larger changes and should be wrapped into a 0.11 release along with the other pending stuff.

Meaning, go ahead and merge, but we commit to no more 0.10.x releases now. And start all the 0.11 cleanup stuff. (When we get to them). Let's get that utils rename in (finally)