"bind" query

Hey @jsccast I hope all is well. I've got another thought to run past you.

I don't want to bloat rulio unnecessarily with a huge feature set, but I am on a minor mission to deemphasize the use of javascript blocks in rules. The next suggestion I have along this line of thought is to introduce a new query type called `bind`. This would be a query that simply takes a JSON map and produces bindings from it, while replacing `?existing_binding` with the existing binding value.

Very often we have the need to "transform" a data structure, or bind things conditionally. Today, we jump into JS to do that. With this `bind` query, hopefully we can avoid the JS and make things more readable.

What do you think?