tee: add conversion impls for Writer

[xtqqczze]

No description provided.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/tail-n0f (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/rm/many-dir-entries-vs-OOM is now being skipped but was previously passing.
Congrats! The gnu test tests/printf/printf-surprise is now passing!
Skip an intermittent issue tests/cut/bounded-memory (was skipped on 'main', now failing)

[oech3]

Why should we have +12 lines just for 2 .into()?

[xtqqczze]

I guess we'll probably extract Writable to uucore::writable.