Create scheduler friendly FS module

[leostera]

Would be useful to have a little File module to read/write files in a scheduler-friendly way.

In short, we would need a module like Net.Socket with functions like:

• File.open file that returns a Fd.t
• File.read fil that return an (Bigstringaf.t, _) result
• File.write file ~data

Internally these functions can call the I/O module to do the actual reading/writing, they just have to beware that if the I/O module says `Retry then you have to call syscall "debugging-name" fd @@ fun fd -> (* do work here *) like this:

let rec read fd ~buf =
  match IO.read fd buf with
  | `Retry -> syscall "file.read" `r fd @@ read ~buf
  (* ... *)

This way we would tell the scheduler we're waiting, and some other process can run.