Investigate why `emulated_add_constant` is two gates more expensive than `emulated_add`

[jtcoolen]

This is just a comment and not a request for a change. I think the way you've done it here is best. However, this is only best because (for some strange reason) emulated_add_constant is two gates more expensive than emulated_add! This is very strange and we should investigate at some point.

Originally posted by @Michael-EY in #75 (comment)