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Update VecDeque implementation to use head+len instead of head+tail (See #99805) This changes `alloc::collections::VecDeque`'s internal representation from using head and tail indices to using a head index and a length field. It has a few advantages over the current design: * It allows the buffer to be of length 0, which means the `VecDeque::new` new longer has to allocate and could be changed to a `const fn` * It allows the `VecDeque` to fill the buffer completely, unlike the old implementation, which always had to leave a free space * It removes the restriction for the size to be a power of two, allowing it to properly `shrink_to_fit`, unlike the old `VecDeque` * The above points also combine to allow the `Vec<T> -> VecDeque<T>` conversion to be very cheap and guaranteed O(1). I mention this in the `From<Vec<T>>` impl, but it's not a strong guarantee just yet, as that would likely need some form of API change proposal. All the tests seem to pass for the new `VecDeque`, with some slight adjustments. r? `@scottmcm`