Remove RefCell::get and RefCell::set

`RefCell::get` can be a bit surprising, because it actually clones the wrapped value. This removes `RefCell::get` and replaces all the users with `RefCell::borrow()` when it can, and `RefCell::borrow().clone()` when it can't. It removes `RefCell::set` for consistency. This closes #13182.

It also fixes an infinite loop in a test when debugging is on.