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....
Is it possible for the `lookahead2()` token to ever be anything other than a punctuator? It's hard to tell from just reading here -- this function has gotten pretty ridiculous.

If so, make sure you pull it out and check the type as well (so as to not match things like string-literals of `"=>"`, for example).