Add missing non-differentiable rules with varargs

[oxinabox]

#252 doesn't include anything that has varargs.

These should be added.
Either by defining the frules and rrules manually,
or by completing JuliaDiff/ChainRulesCore.jl#212

Assuming the latter is done, the missing lines are:

@non_differentiable *(::Union{Regex, AbstractChar, AbstractString}...)
@non_differentiable abspath(::AbstractString...)
@non_differentiable gensym(::String...)
@non_differentiable joinpath(::AbstractString, ::AbstractString...)
@non_differentiable normpath(::AbstractString...)
@non_differentiable print(::Any...)
@non_differentiable println(::Any...)
@non_differentiable string(::Any...)
@non_differentiable write(::IO, ::Any...)
@non_differentiable isassigned(::Any...)
@non_differentiable isblockdev(::Any...)
@non_differentiable ischardev(::Any...)
@non_differentiable isdir(::Any...)
@non_differentiable isfifo(::Any...)
@non_differentiable isfile(::Any...)
@non_differentiable islink(::Any...)
@non_differentiable ismount(::Any...)
@non_differentiable ispath(::Any...)
@non_differentiable eachindex(::Any...)
@non_differentiable xor(Any...)

@non_differentiable Broadcast.combine_styles(::Any...)

@non_differentiable Core.print(::Any...)
@non_differentiable Core.println(::Any...)

@non_differentiable rand(::Any...)
@non_differentiable randn(::Any...)
@non_differentiable randexp(::Any...)

#252 includes all of these manually splatted to betwen 3 and 5 entries, which should be deleted

[oxinabox]

@non_differentiable eachindex(::Any...)

[gxyd]

#252 doesn't include anything that has varargs.
These should be added.
Either by defining the frules and rrules manually,
or by completing JuliaDiff/ChainRulesCore.jl#212

JuliaDiff/ChainRulesCore.jl#212 has been fixed, probably this can be taken up.

[niklasschmitz]

Hey, I'd be happy to try a PR.