std: move io module out of pal, get rid of sys_common::io

[joboet]

Part of #117276.

This does two related things:

1. It moves the platform-specific definitions for IoSlice, IoSliceMut and is_terminal out of pal and into sys and unifies some of them.
2. It gets rid of sys_common::io, moving the non-platform-specific test helpers into std::test_helpers and the buffer size definition to the new sys::io module.

[rustbot]

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[Noratrieb]

sorry for the long review time, but this looks very nice :3
@bors r+

[bors]

📌 Commit f634d2d has been approved by Noratrieb

It is now in the queue for this repository.

[bors]

☔ The latest upstream changes (presumably #136572) made this pull request unmergeable. Please resolve the merge conflicts.

[joboet]

@bors r=@Noratrieb

[bors]

📌 Commit 7433ba6 has been approved by Noratrieb

It is now in the queue for this repository.