Functoria `Key` - why not use standard cmdliner?

Dear everyone,

we have the split between configure time and runtime arguments, and still there's this big Key module, featuring "Terms for cross-stage arguments." -- are these still needed?

Can't we use the normal Cmdliner library in Functoria/Mirage now? And there have the set of keys relevant for configuration time (and then the existing Mirage_runtime.register_arg stuff for runtime keys)?