…upled anyway

… by many-seeds

replace 'locally built rustc' instructions by 'Miri in rustc'

refactor scheduler

Refactors the scheduler to use something akin to a generator -- a callback that will be invoked when the stack of a thread is empty, which has the chance to push a new stack frame or do other things and then indicates whether this thread is done, or should be scheduled again. (Unfortunately I think we [cannot use actual generators](https://rust-lang.zulipchat.com/#narrow/stream/213817-t-lang/topic/Generators.20that.20borrow.20on.20each.20resume.3F) here.) The interpreter loop is now a proper infinite loop, the only way to leave it is for some kind of interrupt to be triggered (represented as `InterpError`) -- unifying how we handle 'exit when calling `process::exit`' and 'exit when main thread quits'.

The last commit implements an alternative approach to rust-lang/miri#2660 using this new structure. Fixes rust-lang/miri#2629.

Bump ui_test crate

All commands can now be revisioned: oli-obk/ui_test#33

Reorganizing `stacked_borrows` in anticipation of a different model

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".

forward verbosity to cargo setup

slight simplifications for borrow tracking

and some renaming for consistency

fix ICE in pointer tracking

Fixes rust-lang/miri#2709

Rustup