Add an `--expand` flag to expand macros and run build.rs scripts

[bnjbvr]

This should get rid of most of the false positives of cargo-machete, hopefully 👀

Perhaps this can be done in multiple steps:

• start with expanding macros using cargo expand; would require a nightly build, likely?
• expand (!) the feature with running build.rs to produce automatically generated code too

Also I think that ideally, there would be a new flag, that can take optional values:

• --expand runs both cargo expand + build.rs generation
• --expand=macros would only do cargo expand
• --expand=build-script would only do build.rs generation