Rename functions in the CloneableVector trait

* Deprecated `to_owned` in favor of `to_vec`
* Deprecated `into_owned` in favor of `into_vec`

[breaking-change]
auto merge of #15725 : aochagavia/rust/vec, r=alexcrichton

* Deprecated `to_owned` in favor of `to_vec`
* Deprecated `into_owned` in favor of `into_vec`

[breaking-change]