Move std::io::Read to alloc::io

[bushrat011899]

ACP: rust-lang/libs-team#755
Tracking issue: #154046
Split From: #156527
Blocked On: #158541

Description

Moves std::io::Read to alloc::io. This is effectively a direct cut and paste, but with a large diff due to how many implementations need to be moved into alloc. Blocked on #158541.

---

Notes

• No AI tooling of any kind was used during the creation of this PR.
• Please see Tracking Issue for alloc::io and core::io #154046 (comment) for a review order and broader context for this PR.

[rustbot]

r? @Mark-Simulacrum

rustbot has assigned @Mark-Simulacrum.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: @ChrisDenton, libs
• @ChrisDenton, libs expanded to 13 candidates
• Random selection from 6 candidates

[bushrat011899]

@rustbot blocked

[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.

[rust-bors]

☔ The latest upstream changes (presumably #158864) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.