Externals provide an ability to bypass module acyclicity

Example

File `A.lama`:

```
import B ;
var x = 1 ;
public fun getX () { x }
printf ("A: %d\n", x)
```

File `B.lama`:

```
external fun getX () ;
printf ("B: %d\n", getX ())
```

Command line:

```bash
lamac -I . -c B.lama && lamac -I . A.lama
./A
```

Result:

```
B: 0
A: 1
```