Add `distinct_at`?

[vnijs]

Many of the _ verb versions have an _at option in 0.7.1. Not so for distinct_. Could this be added?

[hadley]

We should add all scoped variants: distinct_if(), distinct_at() and distinct_all().

[romainfrancois]

@lionel- I can take this one if you like

[lionel-]

Sure thanks!

[lock]

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