New check: Constant only ever appears in distinct statements

[AlexLandau]

Such a distinct statement would do nothing, and this would have caught the error in point_grab: ggp-org/ggp-repository#33

This could probably be extended to constants only appearing in distinct statements and/or negated sentences. Should this be extended to any constants that don't appear in true sentences or the heads of rules? (Note that we should probably restrict this to constants in the inside of function or sentence bodies and have separate checks for sentence or function names. Though a check for function names probably also needs to be added.)