Add client.AddConstraint benchmarks

[willbeason]

Prerequisite: #151

Add benchmarks for adding Constraints. (client.AddConstraint)

Adding Constraints is unlikely to be a bottleneck for normal use cases, but we should have these just so we're aware.

Use Constraints for the ConstraintTemplates chosen to resolve the above issue, as long as their Constraint parameters are of differing complexity. If they are of the same (or very similar) complexity, make up a ConstraintTemplate which uses a complex set of parameters.

[willbeason]

Done!