Add RandomAccessList

[melrief]

Add purely functional random-access list as described in this paper by Okasaki. The random-access list support all the list operations in time O(1) and lookup/update in O(log n).

I have an implementation and I think I can add it to Cats.

[kailuowang]

That sounds a great idea. For some context, we are thinking about creating a new cats-collection project. The main benefit of having this in that new project is that it would allow its API to evolve over a couple of releases. In cats-core things are more rigid due to binary compatibility requirements.

[LukaJCB]

Related: #2058

[kailuowang]

I just assigned #2058 to myself. Hopefully, we'll get a cats-collection going soon.

[kailuowang]

Update: The collection library dogs becomes cats-collections thanks to Stew O'Connor's donation.
We need to finish typelevel/cats-collections#105 (any volunteers?) then we shall be ready to add this there.

[kailuowang]

Cats-collections completed migration and is officially an external cats module now. @melrief if you are still interested in writing a Scala RandomAccessList, it is ready for you.