herd C: better figures.

This includes:
- Merge edges form identical source and target, as cppmem did.
  Controled with the -edgemerge option.
- Draw some edges as symetric (applies to dr). Controled by the
  -symetric <name>,...,<name> option
- Complete cpp11.cfg
Merge branch 'master' of github.com:herd/herdtools
There is a warning 'unused variable', I do not correct.
Conflicts:
	herd/Pretty.ml