refactor: move IO.Channel and IO.Mutex to Std.Sync

[hargoniX]

This PR moves IO.Channel and IO.Mutex from Init to Std.Sync and renames them to Std.Channel and Std.Mutex.

Note that the original files are retained and the deprecation is written manually as we cannot import Std from Init so this is the only way to deprecate without a hard breaking change. In particular we do not yet move Std.Queue from Init to Std both because it needs to be retained for this deprecation to work but also because it is already within the Std namespace and as such we cannot maintain two copies of the file at once. After the deprecation period is finished Std.Queue will find a new home in Std.Data.Queue.

[leanprover-community-bot]

Mathlib CI status (docs):

• ✅ Mathlib branch lean-pr-testing-6282 has successfully built against this PR. (2024-12-02 11:33:38) View Log