Refactor optimize

Refactoring of visitors in optimize.c to simplify (perhaps) and remove boiler plate code. A lot of the visitors uses the exact same code pattern to optimize and check for errors.

In commit baef3a1 a fail compilation test fails but the change should provide a more correct solution. Both lhs and rhs should be checked for errors in a "foo || bar" expression.
Yepp, it should, good catch. I assumed Walter meant that. So for BinExp it will be:
`return !(hasError(e->e1) || hasError(e->e2));`