refactor libcollections::List, add documentation

[erickt]

This PR refactors libcollections::List to remove the @ and replace it with unique pointers. I did this for two reasons. First, none of our other libcollections data structures attempt to share any data between copies. I feel we should instead investigate a separate library for persistent data structures instead of just having this one standing out on it's own.

Second, there is only one user of List, and it's libarena, and this change gets rid of an reference counted pointer in it's structure. It's minor, but it may make it a teensy bit faster.

While I was here, I added a bunch of documentation and extended the api to be a little more consistent with the rest of the sequence data structures. I also removed an old purity test that is now redundant.

[thestinger]

If it's not a persistent list, then I don't really see a point of having it. Why not just use a vector in the arena?

[alexcrichton]

This type/collection seems to have been limping along for quite some time now. It's never really sat well with me, no matter what the api we expose from it is. This may be indicative that it should just be removed.

Do we have any real world use cases of this type?

[erickt]

I'd be fine with us removing List. We need to figure out a good strategy for persistent data structures.

[alexcrichton]

We decided in today's meeting that this type should just get removed for now.

@erickt, do you want to amend this PR? If you're running low on time, I'd be willing to take over!

[erickt]

Closing in favor of #13183.