myria: hash after singleton
myria: hash after singleton

We've had several Myria bugs resulting from the way
that Singleton operators are handled. The easiest way
to handle it, for now, seems to be to just always hash
them out. (Singletons do have a single, non-null column
in MyriaX semantics).

Fix uwescience/myria#704

uwescience/raco,uwescience/raco,uwescience/raco,uwescience/raco,uwescience/raco