JuliaDiff/ChainRules.jl: v0.8.0

Diff since v0.7.70

Closed issues:

• Add tests for new special functions and nan math (#142)
• add derivatives for exponential integrals (#292)
• Drop SpecialFunctions dependency (#319)
• Numerical precision of primal not respected for SpecialFunctions.jl (#402)
• Should we be reexporting ChainRulesCore.jl? (#415)

Merged pull requests:

• Replace NO_FIELDS by NoTangent() (#417) (@mzgubic)
• do not reexport ChainRulesCore (#424) (@mzgubic)
• remove NanMath and SpecialFunctions.jl (#425) (@mzgubic)