[MLton] Option to show path map

[Stephen Weeks]

> I must say that I usually find the requirement to give a file name for
> -show-basis to be a pain.

The argument that was made way back when was that the output was large
and was typically worth saving.  There is also the argument that one
might want to do both -show-basis and -show-def-use simultaneously
(which could be useful for a large program that takes a long time to
elaborate).

> I'd vote for -save-{basis,def-use}.  As -show-{anns,path-map} are mutually
> exclusive (the first one exits), it might be better to have a single option
> 
>   -show {anns|path-map}

Both sound good to me.