Reorganizing stacked_borrows in anticipation of a different model

[Vanille-N]

These commits reorganize all the code inside the former stacked_borrows module and extract the part that really is specific to Stacked Borrows inside the borrow_tracker/stacked_borrows submodule.
Everything not specific to SB is put in borrow_tracker/mod.rs.
This is so that the future Tree Borrows model can be later added as a second submodule and reuse all the contents of borrow_tracker/mod.rs.

This reorganization is accompanied by renamings, mostly from "stacked borrows" to "borrow tracking".

[RalfJung]

Please don't include any merge commits in PRs. If there is a conflict, do a rebase instead.

[bors]

☔ The latest upstream changes (presumably #2694) made this pull request unmergeable. Please resolve the merge conflicts.

[RalfJung]

I left a ton of comments, but most are minor. :)

[RalfJung]

@rustbot author

(call rustbot ready when this is ready for another round of review)

[Vanille-N]

@rustbot ready

[RalfJung]

There are also still some unresolved comments from the previous round. (Github might hide them for you, be sure to click "load more" until that button disappears.)

[bors]

☔ The latest upstream changes (presumably #2699) made this pull request unmergeable. Please resolve the merge conflicts.

[RalfJung]

@rustbot author

[Vanille-N]

@rustbot ready

[RalfJung]

Looks good. :) Just a few final nits. Then please squash this into one (or a few) commits and we are good to go!

[RalfJung]

(force-pushes do not always generate GH notifications, so make sure to always post a note in the PR if you want another review)

[RalfJung]

Thanks!

@bors r+

[bors]

📌 Commit 7cae2e9 has been approved by RalfJung

It is now in the queue for this repository.

[bors]

⌛ Testing commit 7cae2e9 with merge cfa28cf...

[bors]

☀️ Test successful - checks-actions
Approved by: RalfJung
Pushing cfa28cf to master...