SimpleIO.IO_MonadFix

MonadFix IO instance

It was causing a universe inconsistency with VST. Until we investigate more we keep this instance in its own module.

From ExtLib.Structures Require Import MonadFix.
From SimpleIO Require Import IO_Monad.

Instance MonadFix_IO : MonadFix IO := {|
mfix _ _ := IO.fix_io
|}.