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.

Start by reviewing the Blockly mutator blocks and the current handling of monoidal operators such as `&`. Determine how automatic connector creation, identity defaults, deletion, and middle insertion would fit together; done would require an agreed design and an implementation plan, since the issue does not name files or tests.

Written by the indexing model from the issue text.