Remove DISTINCT inside IN/EXISTS/set operations

[roji]

When a subquery is in IN or EXISTS, DISTINCT has no effect and can be removed. The same is true of all set operations, with exception of Concat.

[ranma42]

I would like to tackle this

[roji]

Sure, go ahead!

[ranma42]

While working on this I noticed that there are other cases in which DISTINCT can be safely removed:

• when Takeing 0 or 1 elements (in general, when limiting to 1 element with no skip; this includes .First[OrDefault]() & co)
• when doing a MAX or MIN over the distinct

I will open other issues on this