Rust update: Unboxed closures and removal of proc()

* `proc()` has been replaced with `move ||`, or sometimes `move|:|` ([#19338](https://github.com/rust-lang/rust/pull/19338))
* `core::iter::Map` contains an unboxed closure ([#19467](https://github.com/rust-lang/rust/pull/19467), specifically [this commit](https://github.com/rust-lang/rust/commit/1646d10edc57ec82536d6253f866084beb69a73e))