[MLton] Option to show path map

[Vesa Karvonen]

Quoting Stephen Weeks <sweeks at sweeks.com>:
[...]
> Maybe that behavior also needs a rethink, and we should have the
> only the following, which always stops after printing the desired
> output to stdout.
> 
>   -show {anns|basis|def-use|path-map}

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.

-Vesa Karvonen