use names 'target' and 'reference' for OF/WITH parameters

[riastradh-probcomp]

...instead of just 0 and 1, with 0 meaning the first one and 1 meaning the second only if there were two specified.

This is just a matter of running through ast.py, parse.py, and compiler.py to rename things. No functional change intended (except maybe to the code generated for calls to bql_mutinf/depprob, which should be equivalent because those are supposed to be symmetric).