Add OwnedStr::into_bytes

My primary use case here is sending strings across the wire where the
intermediate storage is a byte array. The new method ends up avoiding a
copy.
auto merge of #8733 : sfackler/rust/str-to-bytes, r=alexcrichton

My primary use case here is sending strings across the wire where the
intermediate storage is a byte array. The new method ends up avoiding a
copy.