Auto merge of #155849 - bushrat011899:io_slice_in_core, r=nia-e

Move `IoSlice` and `IoSliceMut` to `core::io`





ACP: https://github.com/rust-lang/libs-team/issues/755
Tracking issue: https://github.com/rust-lang/rust/issues/154046
Related: https://github.com/rust-lang/rust/pull/152918
Related: https://github.com/rust-lang/rust/pull/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 rust-lang/rust#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`.

---

## Notes

* This PR overlaps with rust-lang/rust#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](https://github.com/rust-lang/rust/pull/155574#issuecomment-4322138454).
* 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.