Improve type stable in function expint

[songjhaha]

We found a type unstable code in expint(z::ComplexF64)

before this PR，the return type is unstable:

julia> @code_warntype SpecialFunctions.En_cf_gamma(1, rand(ComplexF64), 1000)
MethodInstance for SpecialFunctions.En_cf_gamma(::Int64, ::ComplexF64, ::Int64)
  from En_cf_gamma(ν::Number, z::Number, n::Int64) @ SpecialFunctions C:\Users\TR\github\SpecialFunctions.jl\src\expint.jl:209
Arguments
  #self#::Core.Const(SpecialFunctions.En_cf_gamma)
  ν::Int64
  z::ComplexF64
  n::Int64
Locals
...
Body::Tuple{ComplexF64, Union{Float64, ComplexF64}, Int64}
...

After this PR:

julia> @code_warntype SpecialFunctions.En_cf_gamma(1, rand(ComplexF64), 1000)
MethodInstance for SpecialFunctions.En_cf_gamma(::Int64, ::ComplexF64, ::Int64)
  from En_cf_gamma(ν::Number, z::Number, n::Int64) @ SpecialFunctions C:\Users\TR\github\SpecialFunctions.jl\src\expint.jl:209
Arguments
  #self#::Core.Const(SpecialFunctions.En_cf_gamma)
  ν::Int64
  z::ComplexF64
  n::Int64
Locals
...
Body::Tuple{ComplexF64, ComplexF64, Int64}
...

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.44%. Comparing base (fdf95ab) to head (913efcb).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #466      +/-   ##
==========================================
+ Coverage   94.40%   94.44%   +0.03%     
==========================================
  Files          14       14              
  Lines        2914     2914              
==========================================
+ Hits         2751     2752       +1     
+ Misses        163      162       -1     

Flag	Coverage Δ
unittests	94.44% <100.00%> (+0.03%)	⬆️

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[inkydragon]

LGTM

[devmotion]

It would be good to add tests for the changes with e.g. @inferred.