Modernize LoKi Primitives

Eliminate the need for Primitives.h to include itself after some pre-processor manipulations. This is done by avoiding the dedicated specialization for the 'void' functors (i.e. the case where there is no argument). Instead, use variadic number of arguments, and make both the case with and without argument a specialization.

In addition, implement Min and Max in terms of the generic BinaryOp -- which implies moving the handling of 3 or more functions into the min and max helpers in Math.h -- and Round in terms of the generic UnaryOp