add a @unique macro

[catethos]

It will be good if we can do something like

table |>
   @unique(:column)

similar to distinct in dplyr

[davidanthoff]

I think the syntax for us would be @unique(_.colName). Then it would all work with non-table sources etc.

One slightly annoying thing is that unique in base has a version that combines projection with it. I'm not a fan of that, so I think we'll just do that differently here... If one wants to also map, one can just combine things with a pipe operator.