Move std::io::Seek to core::io

[bushrat011899]

ACP: rust-lang/libs-team#755
Tracking issue: #154046
Split From: #156527
Blocked On: #158539

Description

Moves std::io::Seek and SeekFrom to core::io. This is a pretty simple migration, but it is blocked by #158539.

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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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