Proposal: Lazy comprehensions

Create a syntax (I'm thinking in `for*`) for allowing comprehensions to produce an Iterator instead of a list, similar to what python does. This would help when the goal is not to map a list, but use the result in another iteration.

### Proposed Syntax

```coffeescript
somethingExpensive item for* item in list
```

### Normal Behavior

```js
(function() {
var i, len, results;
results = [];
for (i = 0, len = list.length; i < len; i++) {
item = list[i];
results.push(somethingExpensive(item));
}
return results;
})();
```

### Proposed Behavior

```js
(function*() {
var i, len;
for (i = 0, len = list.length; i < len; i++) {
item = list[i];
yield somethingExpensive(item);
}
return results;
})();
```

I'm planning on implementing that (as it does not seem hard, and you'd be a nice introduction to CoffeeScript architecture) so I wanted to know if it's a welcomed feature.

The issue names no source files or tests. Start by reviewing the CoffeeScript compiler architecture and confirming the proposed for* syntax and iterator semantics with maintainers; done means an accepted implementation that produces iterators for lazy comprehensions without changing normal comprehension behavior.

Written by the indexing model from the issue text.