Using a Pkg extension to reduce package load time caused by StaticArrays

[oschulz]

Most of the load time of ForwardDiff is currently due to StaticArrays:

julia> @time_imports import ForwardDiff
      6.8 ms  IrrationalConstants
      0.7 ms  DiffRules
      2.8 ms  StaticArraysCore
      2.1 ms  DiffResults
     17.5 ms  Preferences
      0.2 ms  OpenLibm_jll
      0.3 ms  NaNMath
      2.7 ms  DocStringExtensions 58.56% compilation time
      0.7 ms  Compat
     66.5 ms  ChainRulesCore
      0.5 ms  ChangesOfVariables
      0.8 ms  InverseFunctions
      0.6 ms  LogExpFunctions
      0.2 ms  JLLWrappers
      3.4 ms  CompilerSupportLibraries_jll
      2.6 ms  OpenSpecFun_jll 85.33% compilation time
     15.0 ms  SpecialFunctions
     28.0 ms  MacroTools
      0.2 ms  CommonSubexpressions
    609.2 ms  StaticArrays
    150.9 ms  ForwardDiff

We could use the new Julia v1.9 Pkg weak dependency feature to massively reduce ForwardDiff's load time by creating extensions for StaticArrays (and maybe for ChainRulesCore as well). I had a look, I think StaticArrays is only used in methods that also have StaticArray as an argument type, ForwardDiff doesn't seem to use StaticArrays for internal required functionality.

[oschulz]

Related to #591 and #599 .

[devmotion]

This was implemented in #628 and #625.

[oschulz]

Sorry @devmotion , no idea how I didn't see this.

[oschulz]

Argh, I was working on an outdated master branch by mistake - sorry again.