Optional fields in schema not allowed in closed struct while trimming.

[hjain98]

Schema:: {
    optional? : string
}
d: Schema 
d: {
    optional: "some"
}

On cue trim fails on this with:

d: field "optional" not allowed in closed struct:
    ./a.cue:7:15

[cueckoo]

This issue has been migrated to cue-lang/cue#236.

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