Compiler should not call model to compute output structdef

[mtoy-googly-moogly]

1. Index qops need to expand * like project and reduce do
2. figure out what is missing from the computable-output structdef ( there are result tags in output structs which the compiler can't produce )
3. Touch the pipeline generation code to ask the fieldpsace for the output struct

The last will be tough because a lot of the API in pipeline construction is shaped around the need to ask the model for the output shape, so to do this properly, there will be some surgery. It would be a BadThing™ to skip that work.