impl Reader/Writer for Vec<u8> and &[u8], get rid of {Buf,Mem}{Reader,Writer}

If we end up removing `Seek` from `MemWriter` as suggested in #15915, there's not much reason left to having an explicit `MemWriter`. Instead, we could directly implement `Writer` on `Vec<u8>`. Likewise, we could do the same with `BufWriter`/`BufReader` and `&[u8]` if we take advantage of the fact that neither of them return the slice, so they can shrink the slice as it is read or written to.
