pipes.rs: simplify trait and match

[oech3]

Split trivial changes from other PRs.

• io::Pipe* is the most common tool at here. So import at top.
• pipe size is same for reader/writer. So using pair for simplicity does not cause any confusion.
• Remove unused Read
• simplify match

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/tail/pipe-f2. tests/tail/pipe-f2 is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/timeout/timeout-group (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Skip an intermittent issue tests/tail/tail-n0f (was skipped on 'main', now failing)

[cakebaker]

Thanks!