This changes Variant::visit() to forward the value returned by the
selected visitor invocation. By perfectly forwarding the returned value,
this allows for the visitor to return by value or reference.
Note that all provided visitors must return the same type - the compiler
will otherwise fail with the message: "inconsistent deduction for auto
return type".
This allows the construction of `Variant<int, int, int>`.
While this might not seem useful, it is very useful for making variants
that contain a series of member function pointers, which I plan to use
in LibGL for glGenLists() and co.
typeid() and RTTI was a nice clutch to implement this, but let's move
away from the horrible slowness and implement variants using type
indices for faster variants.
Also adds an AK::Empty struct, because 'empty' variants are useful, but
this implementation leaves that to the user (i.e. a variant cannot
actually be empty, but it can contain an instance of Empty - i.e. a
byte).
Note that this is more of a constrained Any type, but they basically do
the same things anyway :^)
