try to preserve local function names for better stack trace

``` ocaml
let g y =
  let h x = x + y 
  in  h
```

generated code:

``` js
function g(y) {
  return function (x) {
    return x + y | 0;
  };
}
```

it would better to have 

``` js
function g(y) {
  return function h(x) {
    return x + y | 0;
  };
}
```
