Support importing rules into a program

[QuinnWilton]

This is preliminary work for defining a standard library, and just boils down to supporting a simple syntax for importing rules into a program. Note that Datalog has no built-in support for code reuse or for any sort of abstraction, and so we'll need to come up with such a system ourselves.

I think a decent starting point would be to essentially have higher-order rules that are parameterized over input and output relations. Invoking them would generate rules that search over the specified input relations, inserting into the specified output relations. This isn't as expressive as we'll want eventually, but does cover everything we require for our initial suite of CRDTs.

More discussion on what exactly this looks like is welcome.

ETA: 2023-12-31