Check api spec inputs

It's pretty easy to return the wrong thing from stuff like operations, so fresh-data should check these types and ensure they are correct and if they're not, throw a more informative exception than the "non-spreadable" type we're sending by default now.

Another option for operations specifically is to allow them to return undefined if they don't have any promises to return. Or to allow them to return a single promise instead of an array.

Start by tracing how API spec inputs and operation return values are handled in fresh-data, focusing on the operations path mentioned in the issue. Clarify whether the intended result is type validation with a more informative exception, support for undefined, support for a single promise, or some combination; done requires an agreed behavior and coverage for the accepted and rejected forms.

Written by the indexing model from the issue text.