Remove @io::Writer from sys/repr/reflect

At the same time, this updates the TyVisitor to use a mutable self because it's
probably going to be mutating state as it goes along anyway.
auto merge of #8820 : alexcrichton/rust/no-io-writer, r=brson

At the same time, this updates the TyVisitor to use a mutable self because it's
probably going to be mutating state as it goes along anyway.