Remove the 'sub' method from 'At' and migrate to 'sub_raw'

[fallible-algebra]

Continuation of #163978, similar scope to #164244.

[rustbot]

Some changes occurred in engine.rs, potentially modifying the public API of ObligationCtxt.

cc @lcnr

changes to the core type system

cc @lcnr

[rustbot]

r? @jieyouxu

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[fallible-algebra]

r? lcnr

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.