allow manually setting wildcard values

[ed255]

Resolve #433

When creating a statement from an operation we take a vector that will define some initial values of the wildcard map.

[artwyman]

This looks good to me. I tried it out on a private branch with my crafting predicates and it seems to work.
I'll defer to someone who knows this code better to be official reviewer.

[dgulotta]

I'd suggest raising an error if the user tries to assign the same wildcard twice, but otherwise it looks good to me.

[ed255]

I'd suggest raising an error if the user tries to assign the same wildcard twice, but otherwise it looks good to me.

Thanks for the suggestion! I've implemented it in 7c4e354