Rename Variadic to Variant

[C0nsultant]

The generic storage used for e.g. Attribute values was initially designed to store arbitrary datatypes, hence the name Variadic was chosen.

During developement it became pretty obvious that a restriction to a finite, predefined set of datatypes is the more sensible way. Since boost/mpark/std variants do exactly that, they have been used all along.

I propose to rename the auxiliary class Variadic to Variant to reflect this decision.

[ax3l]

Agreed!