myrialang: compile temp relations to new encodings

@7andrew7 pointed out that hardcoding the Relation key in the JSON is probably
a bad idea. Instead, I made the Java side handle that, if at all.

Requires uwescience/myria#564
Do we really need to handle all these cases?  This implies that the while condition could be a store temp, a store, or an ordinary (non-root) operator.  I would prefer to handle the case(s) we expect and assert on everything else.