Overloaded local mutable variables

In our System C implementation, we have builtin support for [local regions](https://se-tuebingen.github.io/oopsla-2022-artifact/casestudies/regions.html):

```
region r {
var x in r = 1;
val closure = box { () => x };
()
}
```

As a next step (in System C or Effekt), we could allow users to overload `var` as follows:

```
effect Variable[T] { def get(): T; def set(value: T): Unit }
def state[R, S](init: Int) { prog: {Variable[S]) => R }: R
def user() {
var x in state = 4;
... x = x + x
}
```
will be desugared to
```
def user() {
state(4) { {x} =>
... x.set(x.get() + x.get()) ...
}
}
```

(I think I have seen something like this in Kotlin)