Static verifications

This issue is mainly a non-exhaustive list of static verification it would be nice to have within OVM.

Depending on the implementation, some might be redundant with others.

• Programs must have a procedure main (i.e: label main ... label end)
• Functions calls must correspond either to runtime functions or to labels of procedures
• Labels must be uniquely defined
• Jumps must go to a label that is defined, and that is not a label of a procedure.
• Jumps can go toward the root of the AST (i.e we can interrupt the evaluation of some expression/instruction, but not jump straight into the middle of one)
• Have a form of (partial) type checking
• Arity checking for functions calls
• Valid usage of arguments i0, i1 ... in in function body (i.e n < arity)
• Check that read accesses are made after a write

Finally, i believe those checks must be performed by default, but we should give the user a way of disabling them (--unsafe ?)