I've changed the commit message to emphasize the transitive implementation over mutable references, as this is the most important part of this change. Making PeekingNext object-safe is well and good, but has far less utility today, as it has no methods that can be invoked through such an object and its super-trait Iterator can be used for dynamic dispatch instead.

I think the most compelling reason to land this is that code like this can easily accept both owned and mutably borrowed iterators:

pub fn sprockets(widgets: impl PeekingNext<Item = Widget>) -> impl Iterator<Item = Sprocket> { ... }

Today, this only accepts PeekingNext types by value, but it is not uncommon to use such types through a mutable borrow. Note that PeekingNext enables the use of functions like Itertools::peeking_take_while, which may be important to both the caller of sprockets and the sprockets implementation.

See also #644, which implements PeekingNext for PeekingTakeWhile. In combination, these changes allow code to generically accept types implementing PeekingNext where Itertools::peeking_take_while can be used by the caller to prepare an iterator and subsequently by a function where restoring items in the originating iterator is important (i.e., the function cannot simply use Iterator::peekable etc., because Iterator::next would unconditionally be called on the originating iterator).

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