auto merge of #13165 : sfackler/rust/io-vec, r=alexcrichton
auto merge of #13165 : sfackler/rust/io-vec, r=alexcrichton

`Reader`, `Writer`, `MemReader`, `MemWriter`, and `MultiWriter` now work with `Vec<u8>` instead of `~[u8]`. This does introduce some extra copies since `from_utf8_owned` isn't usable anymore, but I think that can't be helped until `~str`'s representation changes.

shepmaster/rand,bhickey/rand,waynenilsen/rand,ebfull/rand,achanda/rand,retep998/rand,GrahamDennis/rand,bluss/rand,arthurprs/rand,huonw/rand