* io: implement MultiReader struct
Implement a MultiReader (InStream) which sequentially read from the
provided readers (InStreams). Return IoError.EOF when all of the readers
are read.
* io: implement MultiWriter struct
Implement a MultiWriter (OutStream). The MultiWriter duplicates its
writes to all the provided writers (OutStream).
* io: implement TeeReader struct
Implement a TeeReader (InStream) which reads from a wrapped reader
(InStream) and writes data to the provided writer (OutStream).
