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.


Sure. Its implementation would look exactly like the one for binary heap. Just replace with `pop` with `pop_max`. The question is, how useful is such a function given that it can be implemented entirely in terms of the other pub functions.