collections: Stabilize Vec

The following methods, types, and names have become stable:

* Vec
* Vec::as_mut_slice
* Vec::as_slice
* Vec::capacity
* Vec::clear
* Vec::default
* Vec::grow
* Vec::insert
* Vec::len
* Vec::new
* Vec::pop
* Vec::push
* Vec::remove
* Vec::set_len
* Vec::shrink_to_fit
* Vec::truncate
* Vec::with_capacity
* vec::raw
* vec::raw::from_buf
* vec::raw::from_raw_parts

The following have become unstable:

* Vec::dedup        // naming
* Vec::from_fn      // naming and unboxed closures
* Vec::get_mut      // will be removed for IndexMut
* Vec::grow_fn      // unboxed closures and naming
* Vec::retain       // unboxed closures
* Vec::swap_remove  // uncertain naming
* Vec::from_elem    // uncertain semantics
* vec::unzip        // should be generic for all collections

The following have been deprecated

* Vec::append - call .extend()
* Vec::append_one - call .push()
* Vec::from_slice - call .to_vec()
* Vec::grow_set - call .grow() and then .push()
* Vec::into_vec - move the vector instead
* Vec::move_iter - renamed to iter_move()
* Vec::push_all - call .extend()
* Vec::to_vec - call .clone()
* Vec:from_raw_parts - moved to raw::from_raw_parts

This is a breaking change in terms of the signature of the `Vec::grow` function.
The argument used to be taken by reference, but it is now taken by value. Code
must update by removing a leading `&` sigil or by calling `.clone()` to create a
value.

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