Move std::io into alloc and core

[bushrat011899]

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ACP: rust-lang/libs-team#755
Tracking issue: #154046
Blocked on: #158548

Description

• Moves tests out of std::io into alloctests now that the relevant items are fully available from alloc::io.
• Adds documentation to alloc::io
• Adds prelude modules to core::io and alloc::io.

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Notes

• No AI tooling of any kind was used during the creation of this PR.

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.