added interval heap implementation and fixed immutslist test code

The interval heap seems to be working. Some more tests are probably needed.

cargo test didn't work for me on immutslist.rs since my version of rustc does not implicitly convert a `[T,..N]` into a `&[T]`. So, I added the necessary ampersands to make it compile.


BinaryHeap allows iterating, but it doesn't forward via Deref, it just forwards the underlying iterator itself. This is of course safe, but it seems... weird. That said, this repo is all about doing weird stuff!

Curious what @aturon and @huonw think of this kind of sorcery.