Add std::io::util

This adds a bunch of useful Reader and Writer implementations. I'm not a
huge fan of the name `util` but I can't think of a better name and I
don't want to make `std::io` any longer than it already is.
Add NullReader and make ChainReader take an iterator
auto merge of #10895 : sfackler/rust/io-util, r=alexcrichton

This adds a bunch of useful Reader and Writer implementations. I'm not a
huge fan of the name `util` but I can't think of a better name and I
don't want to make `std::io` any longer than it already is.