Why is `std::vec::Drain` not `TrustedLen`?

https://doc.rust-lang.org/std/vec/struct.Drain.html

As far as I can tell, all Drain iterators are ExactSizeIterator but not TrustedLen.

I went ahead and made a PR since this seems like just an oversight and TrustedLen is unstable anyway: https://github.com/rust-lang/rust/pull/66759

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