Move std::io::copy internals to alloc::io

[bushrat011899]

ACP: rust-lang/libs-team#755
Tracking issue: #154046
Split From: #156527
Blocked On: #158547

Description

Moves the generic implementation of std::io::copy to alloc::io. This is effectively a direct cut and paste. Blocked on #158547.

Worth considering is how we may want to expose the generic copy function for end users. Currently, std only exposes std::io::copy, which internally uses kernel_copy and falls back onto generic_copy as required. We could expose generic_copy as alloc::io::copy, but then alloc::io::copy and std::io::copy would be different functions. There are instances where that already happens (core::panic! is not the same as std::panic! for example), but this would be an easy mistake to make with performance implications.

Alternatively, EII could allow the kernel_copy function to be provided to alloc, so the full copy function is defined in alloc::io and then reexported in std::io. But that's dependent on EII being more stable.

For now, this PR just moves generic_copy to signal that the full std::io::copy is very close to being able to be moved into alloc::io.

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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.

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[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.