v0.1.367 — sep.join(d) over a dict joins its keys (PMAT-668)

Correctness (PMAT-668): ",".join(d) over a dict now joins its keys (like Python) instead of emitting d.join(...) on a HashMap (E0599); the join argument is materialized to the dict's keys, the same as ",".join(d.keys()). Set-join also works as a bonus. Found by the HUNT-V4 differential hunt; verified vs python3. (Multi-key join order is the deferred PMAT-537 dict-iteration-order limitation.) 425 e2e fixtures.