Introduce a pass that analyses if locals need to be mutable#5

At the moment all locals are considered mutable. We should introduce a pass that analyses if we actually modify them. Parameters need to be treated slightly differently here from actual locals, as actual locals might be in a situation where they are assigned once and thus don't need to be mutable, while parameters always need to be mutable when they are being assigned to. In case of a single assignment to a local, we also need to ensure the default value of 0 is not being read beforehand. If that's the case, then we still need to mark it as mutable. This actually sounds like a really complicated pass by now.