Support to AQL functions involving subquerys

[tarsislima]

There is currently no support for subqueries inside functions because second parameter of LET method expect AqlGen Object to wrap the subquery with parentheses
as for example:

// expected 
// FOR l IN locations
//   LET users = FIRST( (FOR u IN users) )

 $mainQuery = AqlGen::query('l', 'locations');  
 $users = AqlGen::query('u', 'users');

// next  produces    
//LET users = (FOR u IN users) 
$mainQuery->let('users', $users);

//but for functions only work with string 
$mainQuery->let('users', 'FIRST((' .  $users->get() .'))');

Some sugestions of implementations?