make the maude backend output more deterministic

@traiansf please review

The Maude backend had some issues that were making the output to maude JVM dependent. I added some code whose purpose is to ensure that the ordering and numbering of rules is the same accross different Java versions.
To make the .out file look nicer, I would search for a pattern <T> <out> ListItem(#buffer(S:String)) </out> </T>.
This would also make it more modular to changes later on; if we decide to drop ListItem or #buffer, then we would only need to change the config.xml file in one place.