Make oven/bake idempotent & public / Remove dump&show as unnecessary

Prior to this change, it was difficult to have multiple outputs with the same ids because the oven would re-generate a number of steps. It also produced awkward commands like `dump&show`, which I never liked.

See `pigpen.oven/bake` for example usage.

Also removed `dump-async` because it was broken & a bad idea to begin with. And cleaned up some type hinting that was doing nothing.