lac: add number template parameter to affine_constraints.cc and refactor

lac: add number template parameter to affine_constraints.cc and refactor

 - Add support for std::complex in internal data structures

 - Refactor special instantiations into .inst.in file.

 - Workaround: add two static_cast<number>(...) to resolve an ambiguity
   when a containerized std::complex is multiplied with another
   std::complex.

 - replace standard comparator for sorting a vector of std::pair with a
   lexicographic ordering where the second component is compared with
   std::abs.

 - templatify and simplify internal data structures

 - simplify internal scratch data (only supports one uniform number type
   now)