Timed references for imperative state

Timed references for imperative state. This module provides an alternative type for references (or mutable cells) supporting undo/redo operations. In particular, an abstract notion of time is used to capture the state of the references at any given point, so that it can be restored. Note that usual reference operations only have a constant time / memory overhead (compared to those of the standard library).

Moreover, we provide an alternative implementation based on the references of the standard library (Pervasives module). However, it is less efficient than the first one.