encoding/openapi: Sort fields when generating structlits

[cuematthew]

As discovered and discussed in https://review.gerrithub.io/c/cue-lang/cue/+/1202688/ the openapi generation seems to be constructing structlits with decls in some unsorted order. The new toposort algorithm always honours the order of decls within StructLit.Decls and so it would seem the openapi generation algs need adjusting, possibly to explicitly call the toposort VertexFeatures.