[MLton] Option to show path map

[Matthew Fluet]

>> I would prefer that we (also) kept the equivalents of the current
>> -show-def-use and -show-basis switches.  The time it takes to process
>> them is small compared to parsing and type-checking and I often put
>> them in build files to be used by default when there is a chance that
>> I'd like to use the data.
> 
> Makes sense.  I don't see any reason not to have both.  And I still
> like the idea of changing -show-{basis,def-use} to -save.

I also like the idea of changing -show-{basis,def-use} to 
-save-{basis,def-use}, especially since we now have
-show {anns,path-map}.