New representation of BATs with macros for simple definition.

Big problem: we have no way to extract names and variables from the
check callback function.

There are two alternative ideas to fix this:
1. Replace the check function with ewffs represented as trees.
   We can extract the names and variables from that tree.
2. Modify the macros so that they add variables and standard names to
   clause specific sets, which are then added to the clause's names and
   variables attributes.