Change `AllocRef::by_ref` to take `&self` instead of `&mut self`
Change `AllocRef::by_ref` to take `&self` instead of `&mut self`


graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust