WIP: Get the `Tree{T}` `prune` and `delete` methods working correctly, and also fix fundamentals of the `Tree{T}` type

[dfellis]

I decided to put this PR up to at least have a record of what's blocked forward progress for now.

I haven't had much time lately to work on Alan. I realized a couple of months ago that the Tree{T} type doesn't work correctly because Alan doesn't have the ability to have references at all. Adding new Node{T}s values would copy the underlying Tree{T} instead of mutating it, which is more threadsafe, but (1) Alan doesn't have threads and (2) is not how people would expect this to work.

This PR has some work to add support for defining a shared value, but it still doesn't quite work right.