API Improvements

[AldaronLau]

• Issues arise when trying to use Task with an internal future type and trying to get it's type (briefly started looking at updating stick crate).

• Also, look at moving the borrow of state, or re-introducing the removed closure in on().

• Maybe implement temporary polyfill for AsyncIterator. Explore the possibility of having types that implement that trait get provided an into_iter() method that returns an iterator of futures (to reduce API duplication).

• Should also add an example depending on whisk, for further API testing before working on wavy crate update.

[AldaronLau]

Last task can be moved to whisk as an issue.