Allow to call functions without assign

We need to allow the next construction:

```
int f(int a) {
     ...
}

int main() {
      f(42);
}
```

For now it forces to write:
```
int f(int a) {
    ...
}

int main() {
    int l = f(0);
}
```

And, which is worse:
```
void f(...) { ... }

int main() {
    void res = f(...); 
}
```
