Remove FillArrays dependency by implementing custom OneElement

[gdalle]

DI source

• Implement custom OneElement
• Use it in basis and multibasis instead of the FillArrays version
• Replace some instances of CartesianIndices with eachindex, which may be faster. Note that CartesianIndices is kept for wrong-mode pushforwards and pullbacks.

DI tests

• Add tests for OneElement and basis
• Remove useless @testset for scenario intactness

DI extensions

• ReverseDiff: Adapt custom basis

---

Load time before:

julia> @time_imports using DifferentiationInterface
      2.7 ms  ADTypes
      0.6 ms  Compat
      0.2 ms  Compat → CompatLinearAlgebraExt
     46.4 ms  FillArrays
      0.2 ms  PackageExtensionCompat
               ┌ 0.0 ms DifferentiationInterface.__init__() 
     31.1 ms  DifferentiationInterface 88.34% compilation time

Load time after:

julia> @time_imports using DifferentiationInterface
      2.3 ms  ADTypes
      0.5 ms  Compat
      0.1 ms  Compat → CompatLinearAlgebraExt
      0.1 ms  PackageExtensionCompat
               ┌ 0.0 ms DifferentiationInterface.__init__() 
     27.2 ms  DifferentiationInterface 87.79% compilation time

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.53%. Comparing base (e89793f) to head (990b637).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #453   +/-   ##
=======================================
  Coverage   98.52%   98.53%           
=======================================
  Files         107      107           
  Lines        4271     4287   +16     
=======================================
+ Hits         4208     4224   +16     
  Misses         63       63           

Flag	Coverage Δ
DI	98.68% <100.00%> (+<0.01%)	⬆️
DIT	98.21% <100.00%> (-0.01%)	⬇️

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