`Stdin:read_line` does not need `&mut self`

Currently, `Stdin::read_line` takes `self` as `&mut`. Why is that? The implementation simply locks and calls `read_line` on the guard, and locking doesn't need a mutable `self`. So this is unnecessary, and the doc doesn't say why this was done. (Nor does the source.)

So I propose for this to be changed to `&self`. Looks like an oversight of some refactoring to me.
