Reinstante `alr with --pin` (#758)

Reinstante `alr with --pin` (#758)

* Implement temporary root copies

* Preparations for manifest modification via Roots

This way, alr with/alr pin become mere proxies for the Alire.Roots functions,
which is as it should have been from the start.

* User_Pins: programmatic creation and TOML oneliner

* Roots manifest editor procedures

* Moved root edition to Alire.Roots.Editable

* Reinstated `alr with [crate] --use`

* Narrowing-down of dependencies without pin

* Tweaks for existing `with` tests

* Re-enabled tests depending on `alr with --use`

* Tweaks found during self-review

* Attempt to fix error in older GNATs