Be less strict about selection-set ordering

For both the `bindings` option, and the soon-to-be-added `typename` option, we validate that two selection sets are equal. Right now we demand that they even have the fields in the same order; ideally we'd allow any equivalent selections. That's surprisingly much work, although it's quite well separated from the rest of the codebase (just modify `selectionsMatch` in `generate/validation.go`).

Start in generate/validation.go at selectionsMatch, then trace how it is used for the bindings option and the planned typename option. Run the existing validation tests before changing behavior. Done means equivalent selection sets are accepted even when their fields appear in different orders.

Written by the indexing model from the issue text.