fixup rules that turn Project into other things

Rename the rules and simplify them to make it more explicit what is going on.
nary_join_rules_test: fix typo
skip nary-join-tests

See #328
Datalog: produce Apply instead of Project

I'm sick of mapping Project to something without Duplicate elimination semantics.
The real solution is to make Datalog emit an Apply instead of a Project, since that
seems to be the desired semantics by the Datalog users. Then the rest of the rules
can make sense.
raco: make all the Project-related rules correct
Merge pull request #329 from uwescience/investigate-bugs-more-carefully

Investigate bugs more carefully