pkg/structs: decide how MinFields and MaxFields should behave with required fields

[mvdan]

From https://review.gerrithub.io/c/cue-lang/cue/+/1194515/comments/4fc29f3a_d942865e:

A question remains whether we should count required fields. With this change they are not counted. (The IsOptional method is inappropriately named, a remnant from when we did not have required fields.) Technically, required fields are not fields. So that means this change is correct. On the other hand, if we have, say, a MaxFields of 1, and we have two required fields, we know there is no completed configuration that can ever satisfy this constraint.

I'm inclined to say that we should count required fields for both MinFields and MaxFields, but this can be left for another CL.