Make Permutations lazy

[Philippe-Cholet]

Related to #792

To make the Permutations adaptor lazy, the operation "prefill the lazy buffer" is now done when handling the initial Start state rather than at definition.

[jswrenn]

This looks great! Can you just add a test to prevent us regressing on this?

For instance, create an iterator whose next call panics, then create a Permutations around it; doing so shouldn't panic.

[Philippe-Cholet]

@jswrenn That's what #792 is for. It helped me make a list of iterators that are not lazy yet and will ensure we don't regress on that front once merged. Make a test now would be soon a duplicate.