Iterators

[lukadev-0]

Add an iterator library akin to Rust's Iterator trait.

Considerations

Working with type packs is annoying as of right now as you can't do something like (T...)? which is why existing libraries don't use type packs (like option, result, future).

This might make it annoying for iterators that produce multiple values, like iterators that produce a key and value for iterating over tables.