Move std::io into alloc and core

[bushrat011899]

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ACP: rust-lang/libs-team#755
Tracking issue: #154046
Blocked on: #155625
Blocked on: #155849

Description

Moves as much of std::io into core::io and alloc::io as practically possible. This PR itself is likely too large to be reasonably reviewed and merged in one go, so I'm dividing it into multiple smaller ones. Each commit in this PR is intended to be as small as possible while still being able to pass CI.

Notable Changes

• Please refer to Move std::io::Error into core #155625 for changes required to facilitate std::io::Error being moved into core::io
• Having Cursor in core::io requires a blanket implemented trait to allow implementation for alloc types, such as &mut Vec<u8>. This is done by WriteThroughCursor.
• Several Read and Write implementations that have been moved to alloc have been altered to account for the no_global_oom_handling configuration option. The only observable change is cases where an OOM event would occur will cause the relevant IO operation to return an OutOfMemory kind of io::Error, rather than deferring to the global OOM error handler.
• Write and Seek are available in core::io, while Read and BufRead are moved to alloc::io. Since BufRead depends on Read, and Read has methods that contain Vec and String in their signature, it would require substantial changes to move everything into core.

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Notes

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

[bushrat011899]

This PR is ready, but blocked on the two PRs mentioned in the description. If/when they are merged, I will split off more commits into other PRs.
@rustbot blocked

[rustbot]

r? @Mark-Simulacrum

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• Owners of files modified in this PR: @ChrisDenton, libs
• @ChrisDenton, libs expanded to 8 candidates