Avoid boxing in Framing (#1247)

* Fix FramingBenchmark

* Add specialized indexOfByte

* Rename indexOfByte to indexOf

* Add missing @since(1.1.0) and reformat scaladoc

* Fix Scala 2.12 ambiguity problem

* Inline nextString

* Change default ByteString#indexOf to call indexWhere

This is always overriden, but I guess it won't hurt to have a default

* Remove default indexOf override

Calling indexWhere is already the default

* Fix MiMa issues

bench-jmh/src/main/scala/org/apache/pekko/stream/FramingBenchmark.scala

@@ -80,7 +80,7 @@ class FramingBenchmark {
     val frame = List.range(0, messageSize, 1).map(_ => Random.nextPrintableChar()).mkString + "\n"
     val messageChunk = ByteString(List.range(0, framePerSeq, 1).map(_ => frame).mkString)
 
-    Source
+    flow = Source
       .fromGraph(new BenchTestSourceSameElement(100000, messageChunk))
       .via(Framing.delimiter(ByteString("\n"), Int.MaxValue))
   }