Create RefReader/RefWriter for io traits

I created RefReader and RefWriter structs that wrap a mutable reference to a Reader or Writer value. This works exactly like the ByRef struct in the iter module and allows passing a reference to a Reader or Writer to function expecting a Reader or Writer by value with the caller retaining ownership to the original value.

I also modified LimitReader to take the wrapped Reader by value instead of by reference.

@sfackler