More sanity checks and static analysis

[maemre]

We need some basic sanity checks to make the language more usable (Prolog has some of these already):

• Checking if clauses of a predicate are together. We should also change our IR to keep separate clauses for more optimizations.
• Checking variables that are used once (hence can be replaced with an underscore).
• Checking if a predicate that is being called is defined.