c582c0c137
Use `append` instead of `extend(drain(..))` The first commit adds `IndexVec::append` that forwards to `Vec::append`, and uses it in a couple places. The second commit updates `indexmap` for its new `IndexMap::append`, and also uses that in a couple places. These changes are similar to what [`clippy::extend_with_drain`](https://rust-lang.github.io/rust-clippy/master/index.html#/extend_with_drain) would suggest, just for other collection types.