Add ExactSizeIterator implementation to Chain

[mendess]

Adds an implementation of ExactSizeIterator for Chain when both iterators implement ExactSizeIterator. Chain already implements TrustedLen this way so I think it makes sense to have this as well.

This change is insta-stable from what I could understand from the docs so it doesn't need an RFC or a ACP from what I could understand from the docs.

This is my first contribution to the standard library that actually changes the API surface of std (specifically core), so sorry if I missed something 😅

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[compiler-errors]

See #101252 (comment)

[the8472]

also #34433, oli might have to increment the counter :D

[mendess]

oops, big sad then. Closing

[Chris00]

It seems this was repeatedly proposed — and I found it desiring the same feature for a particular use case. I was wondering why the contract for len() is not: the exact length unless it is equal to usize::MAX in which was it means >= usize::MAX. With such a change, infinite iterators could also be considered exact size which would be useful at times.