myrialang: re-enable distinct on project

We disabled this a long time ago when we were using Datalog as the
only demo language. Now we have MyriaL and SQL where it is easy
and natural to distinguish between set and bag semantics. Thus, add
set semantics back to the Project operator, which fixes Datalog.
Merge pull request #215 from uwescience/fix-datalog-myrialang

myrialang: re-enable distinct on project