Multiple features (update backend to 2.0) - BREAKING

[MilesCranmer]

Features:

• n-ary operators
• support for user-passed "guesses" of equations
• Mooncake and Enzyme support as autodiff backends
• feature node mutation

This also fundamentally changes some of the mutation operators to support the new n-arity operators. For example, a "rotation" of a binary tree has been generalised in such a way that its behavior changes on binary trees as well.

Similarly, the sampling patterns for other mutations has been changed, so as to improve the uniformity of mutations.

TODO:

• Do we have to use safe operators in the guesses? Or is that not needed?
• Can we do complexity_of_operators for operators that have safe versions?
• Add examples to docs for n-arity operators
• Add some 3-arity operators to the docs and to the builtin sympy mappings (muladd, max, etc.)
• Get working with pytorch/jax export?

[codecov]

Codecov Report

❌ Patch coverage is 92.92035% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.58%. Comparing base (c64ab94) to head (064e201).
⚠️ Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
pysr/sr.py	94.33%	6 Missing ⚠️
pysr/julia_extensions.py	60.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #999      +/-   ##
==========================================
+ Coverage   93.55%   93.58%   +0.03%     
==========================================
  Files          21       21              
  Lines        1566     1638      +72     
==========================================
+ Hits         1465     1533      +68     
- Misses        101      105       +4     

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