evaluator: use toposort by default

[myitcv]

This is a tracking issue that captures the need for us to specify the order of fields during evaluation. This is a necessary part of work on the new evaluator.

spec: need to specify iteration order of struct comprehension · #474 is one aspect of the detail of this issue.

[mvdan]

@cuematthew @mpvl I believe we forgot to update this issue with CUE_EXPERIMENT=toposort (#3558), as I understand that we are resolving this issue in the implementation by providing toposort as the new default and documented field ordering. In my opinion, we should be able to mark this issue as resolved once the default is flipped - and toposort is documented for end users at a high level.

#474 then still tracks updating the spec with any of this, if we wish to.

[mvdan]

After a brief chat with @myitcv, we agreed to repurpose this issue to track enabling the experiment by default, but otherwise we agree that this is pretty much done with Matthew's work.

[mvdan]

@cuematthew resolved this in https://review.gerrithub.io/c/cue-lang/cue/+/1207475, but we failed to link to this issue via a "Fixes". Closing as resolved now.