Add allunique and improve nonunique and describe

[bkamins]

Fixes #3205

Requires adding:

• NEWS.md entry (after other 1.5 release PRs are merged)
• rebasing after fix select bug with copycols=false on SubDataFrame #3231 PR is merged

[bkamins]

Potentially allunique could fail fast, and now it processes all rows, but I guess this should be fast enough in practice (as when doing this test we usually expect it to return true) and doing the faster version would require much more code.

[bkamins]

Thank you!