semantic.AllLabels is not safe

[nathanielc]

See https://github.com/influxdata/flux/blob/master/semantic/label_set.go#L9

It is possible to define a label of -all- on an object once we support string literals as object labels.

We can no longer use a sentinel value unless we use an non UTF-8 sequence which I am not sure is safe.

We need to find a solution to this problem. Originally we tried to use nil as the sentinal value for the entire array, but that was extremely error prone. Perhaps LabelSet needs to be a struct.

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