Be less strict about selection-set ordering

[benjaminjkraft]

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).