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 :^)
