make the constructorof fallback method non-@generated

I don't see a justification for using `@generated`. The effects are
good without it:

```julia-repl
julia> Base.infer_effects(constructorof, Tuple{Type{Int}})
(+c,+e,+n,+t,+s,+m,+u,+o,+r)
```

Updates JuliaObjects#21

src/ConstructionBase.jl

@@ -26,7 +26,7 @@ for (name, path) in [
     end
 end
 
-@generated function constructorof(::Type{T}) where T
+function constructorof(::Type{T}) where T
     getfield(parentmodule(T), nameof(T))
 end