switch to Unix filedesc in Primus Lisp IO module

[ivg]

As a quick hack, we were using OCaml channels to model file objects in our runtime. It was causing immediate issues (#1049) and will cause more (as now the state of the file is separated into two independent and opaque to us states, so interfaces such as seek et al couldn't be easily modeled).

Therefore, my suggestion, as the start for discussion, would be using Unix file descriptors (probably with some other meta information) instead of channels. It is not an issue of immediate concern, but eventually, we will have to implement it.