Some implementations of `Write` should implement `Write` for an immutable reference as well

[nagisa]

The implementations of Write which are MT-safe under the covers can implement a trait along the lines of

impl Write for &'_ Self { ... }

and some already do:

impl<'a> Write for &'a UnixStream { ... }
impl<'_> Write for &'_ TcpStream { ... }
impl<'_> Write for &'_ File { ... }

However, these appear to be the extent of the current implementations. The following types could also do the same:

• Sink – does not inspect data and Write methods are a no-op;
• Stdin/Stdout – internally a Mutex (and even if the Mutex was removed in the future, the underlying syscalls are MT-safe – much like they are for a regular File);
• ChildStdin – similar to File or *Streams;