Initial release of `raco-exe-multitarget`

The program works for me, but it's still relatively young and may not be
fit for some use cases. In particular, binaries are generated with `raco
exe`, not `raco dist`, so the tool won't work for more complex
dependencies.

See `README.md` for the current documentation. I plan to write Scribble
documention when the tool has received some feedback and possibly
resulting changes.