[MLton] Option to show path map

[Stephen Weeks]

> What about the old Unix standard of using the file name - to mean stdout?
> I.e., the option
> 	-show-basis -
> would mean to write the basis to stdout.

I have no objection.  But the semantics of -show-basis would still be
to continue compilation, which could have the surprising side effect
of generating an executable.  I almost always call -show-basis as
follows:

  mlton -show-basis z.basis -stop tc z.mlb

Maybe that behavior also needs a rethink, and we should have the
only the following, which always stops after printing the desired
output to stdout.

  -show {anns|basis|def-use|path-map}