Fixed some stuff in hack branch

Fixed some fatal issues in hack lang.
okay, so then I think it's a problem in hh_client.
In file "/tmp/hh_server_steel/hhi_3bfcbe94/stdlib/builtins_array.idl.hhi"
```
function array_filter<Tv>(Container<Tv> $input, ?(function(Tv):bool) $callback = null) { }
```
array_filter expects function not string, It's not letting my develop further because of this fatal error.