SpanVar, POS, NER, LabeledVar simplifications

SpanVar infrastructure has been simplified, and now includes a non-mutable counterpart.
LabeledVar has been simplified, not distinguishing between "WithTarget" varieties and the others.
As requested: NER representation is not necessarily always "Labeled".  Part of this change, renaming "NerLabel" to "NerTag" and "LabeledNerTag".  ("Tag" now means some CategoricalVar associated with a Token.)
Similarly PosLabel has been renamed PosTag, which better matches common usage.
There is not yet a separated PosTag and LabeledPosTag, but this change is getting big and I'd like to get it merged before there is more divergence.
Once merged, I'll also re-visit work on Phrase, Mention and Relations.