Changes supporting the new ThOr::Combiner<T>

This works together with

• LHCb!2668 (merged)
• Phys!749 (merged)
• Moore!583 (merged)

Improvements:

• ThOr functor framework:
  • Generated C++ code prefers operatorOP( a, b ) to a OP b to avoid some expressions being parsed in surprising ways
  • Generated C++ code expresses integer constants as std::integer_constant, enabling more compile-time dispatch
  • Allow Python-side functors to opt into having multiple positional arguments, enabling e.g. DOCACHI2(1, 2)
• Extend the MakeZip algorithm to optionally wrap its output up in a specific type, such as a variant.
• Forward Sel::Utils::*variant* helpers to LHCb::*variant* ones
• Make ThOr [MAX]DOCA[CHI2]CUT functors more consistently named
• Add SIMD-friendly versions of calculations such as docachi2
• Rework Sel::ParticleCombination to support variant types

Removals:

• Remove usage of the now-dropped unwrapping feature of LHCb::Pr::Zip
• Remove CombineTracks algorithm, replacing it with CombineTracksSIMD in the few places it still existed