Use anonymous_slot_expression instead of range_expression for extra_slots

[sneakers-the-rat]

I had figured that range_expression would be suitable for expressing some subschema for allowable extra properties, but it turns out that range_expression -> anonymous_slot_expression does not actually have the range property. It seems like what we want is some anonymous_slot_expression, which works when i am monkeypatching the metamodel to implement it

[sneakers-the-rat]

i thought this was a valid use of slot_usage, but it may be the case that you can't alias a slot because the aliased slot then can't be found... by the new alias....