Prepare ground for nested structs

[gdalle]

Core

• Add Functors.jl dependency
• Use true as a non-promoting 1 value for gradient as pullback

Test

• Go grab some functions with get_extension(...).test_blabla instead of defining empty stubs in DifferentiationTest
• Separate error-free testing from the JET extension
• Implement recursive isapprox
• Clarify scenario constructor
• Start nested scenarios

Extensions

• Move correctness testing to its own extension which requires Zygote + ForwardDiff (in anticipation of arbitrary structs)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 98.92086% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 93.85%. Comparing base (81d2e83) to head (20e3c53).

Files	Patch %	Lines
...tExt/DifferentiationInterfaceCorrectnessTestExt.jl	98.80%	1 Missing ⚠️
src/DifferentiationTest/test_differentiation.jl	93.33%	1 Missing ⚠️
src/utils.jl	94.44%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #95      +/-   ##
==========================================
+ Coverage   93.00%   93.85%   +0.85%     
==========================================
  Files          53       56       +3     
  Lines        1243     1335      +92     
==========================================
+ Hits         1156     1253      +97     
+ Misses         87       82       -5     

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