feat: remove unmaintained deps

[timsaucer]

This PR is intended to remove these crates that are no longer maintained and show up as a risk when using cargo deny:

• structopt
• generational-arena
• paste

For structopt I've switched it over to clap, it's successor.
for generational-arena I've switched to slotmap, the most widely used replacement I know of.
For paste I've removed it and replaced the macro code using only rust default stable code.

[timsaucer]

@rodrimati1992 would you be willing to review these dependency updates and make a new release? From the project's commit history, it looks like it hasn't had any commits in a few years. Do you have plans to do any further maintenance on it or has the project reached it's final state? We're trying to decide if we should continue to use abi_stable and some of the transitive dependencies are getting stale.