Allow destructuring pattern in arrow functions passed as arguments or on LHS

It didn't work for arrow functions on the rhs or passed as argument. I.e. the following constructs threw a syntax error:

```
var x = ([a, b]) => 42;
foo(([a, b]) => 42);
```

Since arrow functions are usually used as argument for other functions, destructuring in arrow functions was useless so far.

Since I have only little experience with parsers, there may be a better way to do this....
Better to be explicit and clear here anyway and do the check somewhere.
(who knows if another node type might come along one day with a 'value' key that could match this unexpectedly)