This allows to tweak via extractor options some aspects of the cargo
configuration:
* the target architecture
* features (including `*` for all, which we must understand whether to
set by default)
* cfg overrides
Integration tests will be added in a follow-up commit.
This makes the clap flags overlay over `Config` entirely derived via an
attribute macro. Also, the `--intputs-file` option is replaced by a more
standard and versatile `@` parameter file mechanism.
