Permit n-argument operators

[MilesCranmer]

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
• 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

[codecov]

Codecov Report

Attention: Patch coverage is 98.20051% with 14 lines in your changes missing coverage. Please review.

Project coverage is 96.68%. Comparing base (294f789) to head (73ad287).
Report is 126 commits behind head on master.

Files with missing lines	Patch %	Lines
src/OperatorEnumConstruction.jl	94.68%	5 Missing ⚠️
src/EvaluateDerivative.jl	93.93%	4 Missing ⚠️
src/Evaluate.jl	98.78%	2 Missing ⚠️
ext/DynamicExpressionsBumperExt.jl	93.33%	1 Missing ⚠️
src/Node.jl	99.00%	1 Missing ⚠️
src/base.jl	98.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #127      +/-   ##
==========================================
+ Coverage   94.63%   96.68%   +2.04%     
==========================================
  Files          33       30       -3     
  Lines        2723     2591     -132     
==========================================
- Hits         2577     2505      -72     
+ Misses        146       86      -60     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.