…me in run_cargo

Metadata::modified works in all platforms supported by the filetime
crate. This changes brings rustbuild a tiny bit closer towards dropping
the filetime dependency.

Some improvements to the async docs

The goal here is to make the docs overall a little bit more comprehensive and add more links between the things.

One thing that's not working yet is the links to the keywords. Somehow I couldn't get them to work.

r? ```@GuillaumeGomez``` do you know how I could get the keyword links to work?

Ensure stability directives are checked in all cases

Split off  rust-lang#93017

Stability and deprecation were not checked in all cases, for instance if a type error happened.
This PR moves the check earlier in the pipeline to ensure the errors are emitted in all cases.

r? ```@lcnr```

Gracefully handle non-UTF-8 string slices when pretty printing

Fixes rust-lang#78520.

…RalfJung

Implement LowerHex on Scalar to clean up their display in rustdoc

Follow-up of rust-lang#94091.

r? ```@RalfJung```

…crum

Use Metadata::modified instead of FileTime::from_last_modification_ti…

…me in run_cargo

Metadata::modified works in all platforms supported by the filetime
crate. This changes brings rustbuild a tiny bit closer towards dropping
the filetime dependency.