[MLton] def-use mode

[Matthew Fluet]

I continue to very much enjoy def-use mode; if anyone hasn't tried it out, 
it is really a great way to navigate code.

One minor feature request: when a buffer has been editted (and/or saved), 
trying to navigate through that buffer yields "Sorry, no information on 
symbol at point."  It would be nice to differentiate the case when the 
def-use information is out of date with respect to the buffer/file, versus 
there being no def-use information for the file.  I've sometimes had too 
many similarly named emacs buffers open at the same time, and vainly 
switch between them looking for the one linked to the def-use info, only 
to discover that right buffer is out of date.