[4.2] Mutators As Closures

I'd also be open to using this fully from the boot, so that we don't introspect method_exists every hit against the mutator. Comments/Ideas welcome.
Line is a bit long, I would do:
```php
if (array_key_exists($key, static::$mutators[get_called_class()])) return true;

return method_exists($this, 'get'.studly_case($key).'Attribute');
```