Add inherit_fds fork action

[talex5]

This allows you to list FDs you want open in the child process, along with the parent FDs they should inherit. For example:

Process.spawn ~sw [
  Process.Fork_action.inherit_fds [
    1, stdout_to_parent, `Blocking;
    2, stderr_to_parent, `Blocking;
  ];
  ...

inherit_fds.ml contains the algorithm for sequencing the operations so that it doesn't overwrite an FD it needs to use later. If the mapping contains cycles then it allocates a single temporary FD to break the cycle. There is a new fuzz test that generates random mappings and then executes the plan, checking the FDs ended up in the right place.