Bypass struct construction in createinstance

[piever]

The default behavior of createinstance(T, args...) does not work if T has only nonstandard inner constructors.

MWE:

struct A
    x::Float64
    y::Float64
    A(x) = new(x, y)
end

By default, we should probably bypass the constructor, like Serialization does, see https://github.com/JuliaLang/julia/blob/e24e2f091ad0f9f30ad85bf448130325a3f5c060/stdlib/Serialization/src/Serialization.jl#L1341-L1372 for an example.