cargo-profdata UI

[japaric]

This issue is about improving the UI of the cargo-profdata subcommand to make it feel more
integrated with Cargo.

I have no experience with profiling so I have no suggestion for this subcommand. But the other
UI issues may provide some ideas.

cc @whitequark

[japaric]

Also cc @emilio

[vadimcn]

A while ago I started writing 'cargo-pgo' crate. That code is now obsolete, but if you are looking for PGO interface ideas, check out the readme.