Eliminate any redundant, unobservable logic from the their default
method implementations.
The observable changes are that `Write::write_fmt` for both types now
ignores the formatting arguments, so a user fmt impl which has side
effects is not invoked, and `Write::write_all_vectored` for both types
does not advance the borrowed buffers. Neither behavior is guaranteed by
the docs and the latter is documented as unspecified.
`Empty` is not marked as vectored, so that `Chain<Empty, _>` and
`Chain<_, Empty>` are not forced to be vectored.
523b9d9428