Reduce roundtrips for distarray functions.

Distarray functions now take 1 roundtrip per engine.
__import__ -> importlib.import_module in functions.py
Fix bug in testing.

DistArray.var/std return a DistArray instead of an int.
Address @kwmsmith's comments.
Merge pull request #380 from cowlicks/dist-func-round-trips

Reduce dist func round trips to 1 per engine.