`peek_read` should be `&mut self`, not `&self`

[AlexApps99]

I'm currently implementing Reader for a newtype over VecDeque.

To implement peek_read, I need to make the VecDeque contiguous with make_contiguous, which mutates in order to make the data a single contiguous slice.

I don't see any reason why peek_read should require being immutable, as every context I have seen it used in is followed by consume.

Would it be OK to change this in a PR?

[VictorKoenders]

👍 I see nothing wrong with peek_read taking &mut self