impl Into<Vec<u8>> for ByteBuf
As BurntSushi observes, there is an `impl<T, U> Into<U> for T where U: From<T>` in libcore. We have `From<Vec<u8>>` for `ByteBuf` since we have implemented `From<T> for ByteBuf where T: Into<Vec<u8>>`, so this is redundant anyway.
