This opens up DynamicExpressions.jl to D-degree nodes via an additional type parameters. This is still a work-in-progress, but I wanted to track the TODO's here.

TODO:

• Write upgrade guide in CHANGELOG.md
• Tests
  • Test string paths
  • Random expression evals for arbitrary arity, compare against Julia evaluation
  • 100% diff coverage (improved coverage; but not 100)
• Performance
  • Evaluation performance (seems to be roughly the same now)
  • Allocation performance (at the moment, it seems to result in double the # of allocations, and 33% greater memory usage)
  • Either split types, or get generic type to match existing performance for
  • Consider using ::Union{Nothing,N} rather than Ref
  • Profile SymbolicRegression.jl against the diff.
    • Need to get DynamicDiff.jl working first.
• Feature coverage
  • Generic operator enum compatibility
  • Derivative compatibility
  • Preallocation compatibility
  • LoopVectorization.jl compat
  • Bumper.jl compat
  • Verify graph node compat
  • Verify simplification compat
• Cleaning
  • Simplify evaluation code with generic Base.Cartesian version
  • Manually verify no internal uses of .l and .r (temporarily set these to be errors)
  • Introduce alias Node{T} = NNode{T,2} for full backwards compatibility