Consider add-on-the-fly mode for monoidal operators?

Blockly's mutator blocks are actually not all that intuitive for adding many arguments to a block like `&`. We should eventually maybe consider more options for adding new connectors automatically as needed: when all existing slots are full, just add a new one, defaulting to the identity value. Then there will be questions about how to delete slots, insert them in the middle, etc... but maybe they can be solved in a way that's more direct than the mutator dialog.