Move quit and exit to builtins.

This closes #77.

Note, however, that this likely will not work from within an imported file (I think it will quit the imported file, not the entire program).  That wouldn't work with the current solution, either.  These changes could be easily modified to catch the SystemExit exception from the runit wrapper and print the message instead of reporting it as an exception.  I felt this was better for now, so as not to place an additional burden on runit.