Move IoSlice and IoSliceMut to core::io

[bushrat011899]

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ACP: rust-lang/libs-team#755
Tracking issue: #154046
Related: #152918
Related: #155625

Description

Moves std::io::IoSlice and std::io::IoSliceMut into core::io. This is required for the Read and Write traits to be moved into alloc and/or core, as they contain stable methods which work with IO slices. Similar to #155574, this PR inlines the std::sys types required to create an ABI compatible type for the platforms where such compatibility is guaranteed.

Additionally, I've moved the relevant tests out of std::io::tests into coretests::io::io_slice.

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Notes

• This PR overlaps with Move OS-dependent parts of std::io to alloc #152918, but goes further than moving the IO slice types to alloc and instead moves them straight to core. Since these types have no interaction with allocation, and doing so will allow Write to move to core::io, I consider this a better home for these types.
• Some discussion around the decision to not use a core::sys module can be found here.
• I've renamed unsupported to generic to better reflect that IoSlice(Mut) is supported by all platforms, it just doesn't have a special ABI-compatible type. I don't want to imply that parts of core "don't work" depending on the target; IoSlice(Mut) works exactly as expected on all targets.
• I've made pub items within each platform-specific representation pub(super) to highlight that everything within core::io::io_slice is an internal implementation detail not meant for any other part of the crate to be aware of.
• No AI tooling of any kind was used during the creation of this PR.

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[lygstate]

a long awating staff, thanks.