use declarative 'clap' syntax

uses the 'declarative' Clap syntax.

it's in beta, but i've mitigated this by pinning the dependency

I think the derive-based declarative style is ultimately more maintainable, and the stringly-typed fluent API irks me.

This is technically a breaking change, since the 'global' args are no longer global. But i don't think they were really working as intended anyway, judging from the comments