[MLton] Option to show path map

[Stephen Weeks]

> I think it would be nice if mlton had a command line option to show
> the (MLB) path map. 
...
> I made an implementation of the option.  See the patch below.

I wonder if, to be consistent with -show-basis and -show-def-use, it
would be better to make it

  -show-path-map <file>

rather than

  -show-path-map {false|true}

And, for consistency, don't exit after writing the data to a file.