Destructuring in lambda function headers

This doesn't work today:

```
...
let x = [(1,2), (3,4)]
List.map((a,b) => a + b, x)
```

but this does:

```
...
let x = [(1,2), (3,4)]
List.map(yy, x)
...
function yy((a,b)) = a + b
```

Would be good to support destructuring in lambda function headers