.balance defined n Flow/Source, to return a SubFlow

ktoso 11:06 what's the plan about Balance being defined on Flow/Source - we don't? // cc @rkuhn @drewhk found myself that it would look nice for demoing "look, it's parallel now"

rkuhn 11:06 .balance should actually give you a SubFlow so that we can materialize that a number of times to exploit parallelism while losing ordering and if it takes a (strict) integer argument for the parallelism then we can actually desugar that into static > junctions