IList API

[cranst0n]

It would be nice to add a few more convenience functions to IList. This issue is kind of a running log of what I've come up against. I'm happy to work on these in the near future.

• [ ] int count(bool Function(A a)) // returns a count of elements that pass the given predicate
• [ ] Option lift(int ix) // returns the element at the given index (if the index exists)
• [ ] List padTo(int len, elem: A) // pad the list (if necessary) to the given length filling in the given element