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.
 * Made `ReflectionFunctionAbstract::getAttributesRecursive()` and `ReflectionFunctionAbstract::getAttributeRecursive()` non-abstract as they are missing from the Zend API and thus userland classes extending `ReflectionFunctionAbstract` will be missing them.