auto merge of #8607 : sfackler/rust/extensions, r=brson

The method names in std::rt::io::extensions::WriterByteConversions are
the same as those in std::io::WriterUtils and a resolve error causes
rustc to fail after trying to find an impl of io::Writer instead of
trying to look for rt::io::Writer as well.

diff --cc src/libstd/rt/io/extensions.rs
Simple merge