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.


Yes, the comment should say more than "item indices...". Though, the corresponding struct definition is somewhat self-explanatory in that respect.