fix: yield expression should be callable

[summivox]

Since yield expressions evaluate to argument passed to next, if a function is passed it should be callable.

Example:

f = ->* (yield)!
g = -> console.log 'hit'
f!
  ..next!
  ..next g

should compile to:

var f, g, x$;
f = function*(){
  return (yield)();
};
g = function(){
  return console.log('hit');
};
x$ = f();
x$.next();
x$.next(g);

[vendethiel]

Please add some tests :).

[summivox]

@vendethiel done

[vendethiel]

Thanks!