cue: tidy up and implement semantics of defaults and references

[mpvl]

CL https://cue-review.googlesource.com/c/cue/+/1660 introduced an alternative semantics for default values that allow also default values to be fully associative, commutative, idempotent, and distributive.

This still needs to be implemented. The implementation should go hand in hand with tidying up and clarifying the semantics of references and the use of defaults in arithmetic and the combination thereof.

[cueckoo]

This issue has been migrated to cue-lang/cue#28.

For more details about CUE's migration to a new home, please see cue-lang/cue#1078.