Forward more ExactSizeIterator methods and `is_empty` edits
- Forward ExactSizeIterator methods in more places, like `&mut I` and `Box<I>` iterator impls.
- Improve `VecDeque::is_empty` itself (see commit 4)
- All the collections iterators now have `len` or `is_empty` forwarded if doing so is a benefit. In the remaining cases, they already use a simple size hint (using something like a stored `usize` value), which is sufficient for the default implementation of len and is_empty.
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