Added setter for Custom Struct lists

As seen [in the tests](https://github.com/glycerine/go-capnproto/blob/master/struct_test.go#L128), special crafting like `C.PointerList(mylist).Set(i, C.Object(myitem))` has to be made in order to set the items of a custom list, since all the other list for built-in data types already support `Set` I thought that custom lists should have it as well.