I was given some guidance[1] on the Rust developer forums that my approach of
implementing Iterator on these types wasn't quite right.
> In general, iterators are distinct data structures than the primary struct
that owns the data, in part due to issues like this, but also due to concerns
around iterator invalidation and the need to carry around iteration state more
generally.
I took this to heart and replace the Iterator implementations with an iter()
method that returns an `impl Iterator<Item=Move>`. This works so much better and
lets me delete a bunch of code!
Additionally, the iter() methods on the piece-wise move generator types return
`impl Iterator<Item=&Move>`, and then the top-level Moves::iter() returns a
.cloned().
Overall I'm really happy with this!
[1]: https://users.rust-lang.org/t/tricky-lifetime-may-not-live-long-enough-error-on-an-iterator-wrapping-a-boxed-iterator/104650/3
Generate moves for kings.
This struct is trying out a different approach to iterators where the type
itself isn't one, but it has an iter() method that returns an iterator over all
the move lists.
This struct also builds all the moves up front, in new() before the new instance
is returned.
