2006: Improvements around `Arc<[T]>` r=matklad a=sinkuu
First commit tries to avoid cloning `Arc<[T]>` to a temporary `Vec` for mutating it, if there are no other strong references. Second commit utilizes [`FromIterator for Arc<[T]>`](https://doc.rust-lang.org/std/sync/struct.Arc.html#impl-FromIterator%3CT%3E) instead of `.collect::<Vec<_>>().into()` to avoid allocation in `From<Vec<T>> for Arc<[T]>`.
Co-authored-by: Shotaro Yamada <sinkuu@sinkuu.xyz>
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