Reflection API: Add ::getFullName() method.

I propose a new method ::getFullName() for all methods that implement \Reflector. (I would add it to the \Reflector interface, but this would be a BC break.) The full name would be, depending on the type: function: "str_replace()", "Acme\foo()". class: "Acme\Animal\Cat", "stdClass". method: "Acme\Animal\Cat::meow()". property: "Acme\Animal\Cat::$color". etc. I don't know if we can cover all reflection API classes in a meaningful way. E.g. for ReflectionObject, the full name would be the same as for the class, otherwise it would get complicated. Currently the ::__toString() and ::export() return something else, which is not really useful. The goal is to have something that is different for different items. E.g. a class and a function with the same name need to have a different "full name", hence the added "()" for functions and methods. As a rule of thumb, the "full name" is what one would put after a @see tag in phpDoc, but without the leading "\\". For consistency with the rest of PHP and reflection API, we should use QN and not FQN as return value. Related: I started another thread proposing a ::getHash() method. I originally though to propose the two things in the same thread, but I now think they should be discussed independently.