Fix built-in output handling for base doc refs

Previously, built-ins were not hanlding references in output positions
properly. Specifically, they were just calling Equals() on the term in the
output position against the built-in function result. The Equals() call would
return false and evaluation would stop.

Now, the built-ins just call the unify function which handles references
correctly and will also work in cases where a composite value (potentially
containing variables and/or references) is given in the output position.