Improved Zend compatibility in the reflection API.

 * `inNamespace()`, `getNamespaceName()`, and `getShortName()` should not be
   `final`. This was causing issues in phpspec which mocks these methods.
 * Added missing `ReflectionFunctionAbstract::getClosureScopeClass()` and
   `ReflectionFunctionAbstract::__clone()`.
 * Removed abstract `ReflectionFunctionAbstract::getClosure()` as it's not in
   the Zend API meaning userland classes extending `ReflectionFunctionAbstract`
   are unlikely to implement it.
 * Made `ReflectionFunctionAbstract::isClosure()` non-abstract as it's not
   abstract in the Zend API.