Remove impl Seek for MemWriterFakeStream.

Its seekability wasn’t being used, and so in light of the changes made
in rust-lang/rust#15915 it should be removed, rather than switching from
the now-unseekable ``MemWriter`` to the new ``SeekableMemWriter``.

This necessitated adding the ``get_ref`` method which was added to
``MemWriter`` at some point.