!str,htc replace and remove OneBoundedInterpreter

main work by @drewhk with contributions from @2m and @rkuhn

This work uncovered many well-hidden bugs in existing Stages, in
particular StatefulStage. These were hidden by the behavior of
OneBoundedInterpreter that normally behaves more orderly than it
guarantees in general, especially with respect to the timeliness of
delivery of upstream termination signals; the bugs were then that
internal state was not flushed when onComplete arrived “too early”.

akka-docs-dev/rst/scala/code/docs/http/scaladsl/HttpServerExampleSpec.scala

@@ -352,7 +352,7 @@ class HttpServerExampleSpec extends WordSpec with Matchers {
         pathEnd {
           (put | parameter('method ! "put")) {
             // form extraction from multipart or www-url-encoded forms
-            formFields('email, 'total.as[Money]).as(Order) { order =>
+            formFields(('email, 'total.as[Money])).as(Order) { order =>
               complete {
                 // complete with serialized Future result
                 (myDbActor ? Update(order)).mapTo[TransactionResult]
@@ -373,7 +373,7 @@ class HttpServerExampleSpec extends WordSpec with Matchers {
         path("items") {
           get {
             // parameters to case class extraction
-            parameters('size.as[Int], 'color ?, 'dangerous ? "no")
+            parameters(('size.as[Int], 'color ?, 'dangerous ? "no"))
               .as(OrderItem) { orderItem =>
                 // ... route using case class instance created from
                 // required and optional query parameters