Auto merge of rust-lang#26126 - Nashenas88:sync-send-libcore-iter, r=…

…huonw

This addresses an item in rust-lang#22709. SizeHint in libcore/iter.rs also implements Iterator, but it's implementation is not accessible and is only used to send size hints to extend (it appears to be a performance improvement to avoid unnecessary memory reallocations). The is the only implementation of Iterator within libcore/iter.rs that is not/cannot be tested in this PR.

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