readme: tag-gc tweaks

r? `@saethlin`

Is this option truly needed often enough that it should be in the 'common' section? If not I vote for moving it to the 2nd section. Also `10,000` is a confusing way to write numbers in an international context (in many languages, `,` in a number plays the role of the point in English number notation, so using a space or underscore is less likely to lead to confusion).

…tics

GC: factor out visiting all machine values

`@saethlin` that is roughly what I had in mind.

I think some parts of the state are skipped by the visitor. I listed the ones that I found in FIXMEs but I am not sure if that list is complete.

run all extern-so tests consistently without dependencies

Don't back up past the caller when looking for an FnEntry span

Fixes rust-lang/miri#2536

This adds a fix for the logic as well as a regression test. In the new test `tests/fail/stacked_borrows/fnentry_invalidation2.rs`, before this PR, we display this diagnostic:
```
help: <3278> was later invalidated at offsets [0x0..0xc] by a Unique FnEntry retag
  --> tests/fail/stacked_borrows/fnentry_invalidation2.rs:13:5
   |
13 |     inner(&mut t);
   |     ^^^^^^^^^^^^^
```
Which is very misleading. It is not this call itself, but what happens within the call that invalidates the tag we want. With this PR, we get:
```
help: <2798> was later invalidated at offsets [0x0..0xc] by a Unique FnEntry retag inside this call
  --> tests/fail/stacked_borrows/fnentry_invalidation2.rs:20:13
   |
20 |     let _ = t.sli.as_mut_ptr();
   |             ^^^^^^^^^^^^^^^^^^
```
Which is much better.

remove macOS work-around that is no longer needed

Judging from actions/cache#403 it sounds like this work-around is not needed any more.

… all dependencies

CI: use cargo sparse registry

CI spends a few minutes downloading the index so this could help.

Add flag to specify the number of cpus

Apparently you can't rename a branch from github's website without it closing all your PRs with that branch. So this is  just rust-lang#2545

…nyway

more details in stacked borrows tag tracking

Expand VisitMachineValues to cover more pointers in the interpreter

Follow-on to rust-lang/miri#2559

This is making me want to write a proc macro 🤔

r? `@RalfJung`