Rollup merge of #66341 - crgl:vec-deque-extend, r=Amanieu

Rollup merge of #66341 - crgl:vec-deque-extend, r=Amanieu

Match `VecDeque::extend` to `Vec::extend_desugared`

Currently, `VecDeque::extend` [does not reserve at all](https://github.com/rust-lang/rust/pull/65069#discussion_r333166522). This implementation still runs a check every iteration of the loop, but should reallocate at most once for the common cases where the `size_hint` lower bound is exact. Further optimizations in the future could improve this for some common cases, but given the complexity of the `Vec::extend` implementation it's not immediately clear that this would be worthwhile.