Arithmetic backends

[foolnotion]

This PR is an important change to the tree evaluation mechanism in Operon, which allows different libraries to be used instead of Eigen.

This brings:

• more flexibility, as potentially more backends/number formats can be more easily supported in the future
• a better API, based on mdspan, which simplifies things in the dispatch table/interpreter
  • we use the reference implementation of P0009 from Kokkos: https://github.com/kokkos/mdspan
  • see this video for an argument why this is good
• new arithmetic backends:
  • Expressive Vector Engine (EVE) github | DOI | video
  • Fastor github | DOI
  • VDT github | DOI
  • Approximate math based on various bit tricks and optimizations at the level of the floating point representation (thanks @christophercrary)

Selection between the backends is done at compile time via the -DMATH_BACKEND option. Valid values are: Eigen, Fastor, Eve, Vdt, Fast_v1, Fast_v2, Fast_v3 (where v1, v2, v3 refers to the precision of the approximation, from least- to most precise)