rules.PushApply: push Apply into Projectingjoin when appropriate
Revert "flink: Override toString, but has no effect"

This reverts commit 60fdaf0c481609c8f491a7444ba3288f4e89970a.

Conflicts:
	raco/language/flink/flink.py
flink: override name for Map and Filter
flink: improve name for filter function
flink: minor cleanup and better errors
flink.rules: add FlinkProjectingJoin
fakedb: implement projectingjoin delegate in myria join
flink: now that we have Flink rules, verify that they are correct
flink: fix a bug