Incorrect code gen using GET DIRECT LOOKUPS lookups

[ToddFincannonEI]

The LOOKUP FORWARD, LOOKUP BACKWARD, and LOOKUP INVERT functions takes a lookup as its first argument. This works fine when the lookup is defined using the literal lookup syntax, as in our existing test. However, when the lookup is read from a file with GET DIRECT LOOKUPS, incorrect code is generated: the lookup is dereferenced with _LOOKUP instead of being passed to the function.

[ToddFincannonEI]

This is solved by handling these three functions as special forms in visitVar. Note that WITH LOOKUP also takes a lookup argument, but this case does not apply there, since the lookup argument is meant to be a literal lookup.

Some function calls in visitVar were unnecessarily repeated many times, so I took the opportunity to refactor it.

I added tests for this case in the directlookups.mdl test model.