Migrate away from `gumdrop` for argument parsing about rage HOT 3 CLOSED

Reading clap-rs/clap#380 it sounds like although there is no built-in support yet, clap does have sufficient APIs to enable localization, and other people have tested doing so with Fluent. Combined with the fact that I already have a clap representation of the CLIs, I think I'll put effort into that direction and see how far I get.

from rage.

Potentially useful comparision data: https://github.com/rosetta-rs/argparse-rosetta-rs

from rage.

#442 migrates to clap, which made it pretty easy (once I figured out the kinks) to localize the usage and help text. I didn't end up using any of the existing builder-based clap representation, as migrating from gumdrop to clap's derive-based representation ended up being simpler. I still need to figure out how to migrate the completions etc. away from my current approach to de-duplicate, but if that works then this is almost certainly what I'll merge.

from rage.