[MLton] Option to show path map

[Stephen Weeks]

> Note that -show-path-map behaves like -show-anns.
...
> It would probably make sense to use a different naming convention
> for such options (or commands), e.g.  -print-anns and
> -print-path-map.

Makes sense.  Or perhaps keep show-{anns,path-map} and change
-show-{basis,def-use} to -save or -write?  I often get confused by the
name "show" and I know others do too.