[MLton] Option to show path map

[Stephen Weeks]

> 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.