Move std::io::pipe code into its own file

[tbu-]

Also update the docs for the new location, create a section "Platform-specific behavior", don't hide required imports for code examples.

[rustbot]

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[joboet]

This is a duplicate of #135583.

As the other one is approved already, I'm closing this one. Feel free to reopen if the other PR does not address everything done here.

[tbu-]

@joboet I can't re-open since a maintainer closed it. Could you reopen this PR? I'll base it on top off #135583.

[rustbot]

This PR modifies tests/run-make/. If this PR is trying to port a Makefile
run-make test to use rmake.rs, please update the
run-make port tracking issue
so we can track our progress. You can either modify the tracking issue
directly, or you can comment on the tracking issue and link this PR.

cc @jieyouxu

[NobodyXu]

#135583 has been merged

cc @tbu- sorry that I didn't see your PR for this, I was planning on also opening a PR for stabilisation, but maybe you can do it within this PR?

[tgross35]

@tbu- could you rebase?

[tbu-]

@tgross35 There appear to be no merge conflicts according to my local git. I included the same commits as #135583, so git should handle it just fine.

[tbu-]

I was planning on also opening a PR for stabilisation, but maybe you can do it within this PR?

I'll open a stabilization PR.

[tgross35]

The merge is conflict-free but GH does not automatically drop commits, so the diff view is incorrect.

[joboet]

Great, thank you!
@bors r+

[fmease]

@bors r- ('fixing' desync)

[NobodyXu]

isn;t this PR akready merged?

[fmease]

Yes but the bors queue is slightly out of sync atm and this merged PR showed up in the "merge queue", I had to kick it out. We have a proper procedure for resyncing the queue but that takes ~45 minutes and I didn't want to do that rn (for several reasons).