Use Generic Tuples to type labels

[breezewish]

trafficstars

variable_labels can be expressed by generic tuples (variables in tuple length).

This brings several benifits:

1. Runtime cost of checking the length of variable_labels can be eliminated.
2. Label length can be checked in compile time.

[siddontang]

can you support this?

[breezewish]

@siddontang Ok. When I'm free I will handle this. By the way this will change interface signatures, which will be a breaking change.