Use Array for byClass reachability info

This reduces the retained size on the test suite for the infos as
follows:

BaseLinker: 26MB -> 24MB
Refiner:    22MB -> 20MB

linker/shared/src/main/scala/org/scalajs/linker/analyzer/Infos.scala

@@ -93,7 +93,7 @@ object Infos {
   )
 
   final class ReachabilityInfo private[Infos] (
-      val byClass: List[ReachabilityInfoInClass],
+      val byClass: Array[ReachabilityInfoInClass],
       val globalFlags: ReachabilityInfo.Flags
   )
 
@@ -374,7 +374,7 @@ object Infos {
       setFlag(ReachabilityInfo.FlagUsedExponentOperator)
 
     def result(): ReachabilityInfo =
-      new ReachabilityInfo(byClass.valuesIterator.map(_.result()).toList, flags)
+      new ReachabilityInfo(byClass.valuesIterator.map(_.result()).toArray, flags)
   }
 
   final class ReachabilityInfoInClassBuilder(val className: ClassName) {