Erasing the traces of the previous version of the module when importing a new version

[per-gron]

One thing that would be really cool to do is to keep track of the globals that are defined within (the run-time instance of) a module, and when a module is re-imported make sure to set all the previous names to #!unbound or something to that effect. That would show some bugs earlier in the process, for instance if you rename a procedure and forget to change all the references.