Upgrade `structopt` to `clap` #993

Job	Run time
Run the in-tree version of cargo-insta on ourselves	46s
Test on Latest (ubuntu-latest)	1m 30s
Check on MSRV	51s
Test on Latest (macos-latest)	1m 15s
Test latest deps	1m 5s
Test on Latest (windows-latest)	2m 41s
Build docs	47s
	8m 55s

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