many changes regarding coercions:

many changes regarding coercions:
- new `prefer coercion foo` command to reorder the list of coercions associated with a src >-> tgt pair
- rewritten coercions undoing mechanism
- coercion composition fixed (generates more coercions)
- librarySync rewritten, procedures to generate derived lemmas (eliminators, invertion lemmas, ...) are hooks registerd by other modules that can thus live anywhere (also in tactics/ like the one for inversion)
- manual fixed to talk about the new command
- coercions graph is drawn together with a list of coercions that makes their precedence visible (and can be altered with the prefer coercion command)
- dump-moo fixed to print coercions
- removed `Coercion attribute in XML files (YOU NEED TO RECOMPILE)