Graphviz: eliminate duplicate arrows

This ensures that at most one arrow is drawn for every source and
destination pair. Duplicate arrows add no information value and only
make the diagram cluttered.

For example, see
https://github.com/kaitai-io/kaitai_struct_formats/blob/e533644618670a3b94be687253b0bf11a051640c/common/bcd.ksy,
which used to generate many duplicate arrows with the same source and
destination. It looks much cleaner now.

shared/src/main/scala/io/kaitai/struct/GraphvizClassCompiler.scala

@@ -8,7 +8,7 @@ import io.kaitai.struct.languages.components.{LanguageCompiler, LanguageCompiler
 import io.kaitai.struct.precompile.CalculateSeqSizes
 import io.kaitai.struct.translators.RubyTranslator
 
-import scala.collection.mutable.ListBuffer
+import scala.collection.mutable.{ListBuffer, LinkedHashSet}
 
 class GraphvizClassCompiler(classSpecs: ClassSpecs, topClass: ClassSpec) extends AbstractCompiler {
   import GraphvizClassCompiler._
@@ -17,7 +17,7 @@ class GraphvizClassCompiler(classSpecs: ClassSpecs, topClass: ClassSpec) extends
 
   val provider = new ClassTypeProvider(classSpecs, topClass)
   val translator = new RubyTranslator(provider)
-  val links = ListBuffer[(String, String, String)]()
+  val links = LinkedHashSet[(String, String, String)]()
   val extraClusterLines = new StringLanguageOutputWriter(indent)
 
   def nowClass: ClassSpec = provider.nowClass