Add initial support for functions wih multiple inputs/outputs as well as multiple functions

- Currently there is no multi-out support for Supplier and I am not really sure what the use case would be so holding off.
- The logic in FunctionConfiguration effectively split in bootstrapping simple functions (e.g., Function<String, String>) vs. multi-in/out (e.g., Function<Tuple2<Flux<String>, Flux<String>>, Flux<String>>). This is temporary given that multi-in/out is still considered WIP. Once it becomes stable we can merge the two two for consistency.
- Added multiple input/output support for TestBinder
- Updated documentation

Resolves #1746
Resolves #1745
Resolves #1314

docs/src/main/asciidoc/preface.adoc

@@ -85,16 +85,17 @@ Modify the `com.example.loggingconsumer.LoggingConsumerApplication` class to loo
 [source, java]
 ----
 @SpringBootApplication
-@EnableBinding(Sink.class)
 public class LoggingConsumerApplication {
 
 	public static void main(String[] args) {
 		SpringApplication.run(LoggingConsumerApplication.class, args);
 	}
 
-	@StreamListener(Sink.INPUT)
-	public void handle(Person person) {
-		System.out.println("Received: " + person);
+	@Bean
+	public Consumer<Person> log() {
+	    return person -> {
+	        System.out.println("Received: " + person);
+	    };
 	}
 
 	public static class Person {
@@ -114,10 +115,10 @@ public class LoggingConsumerApplication {
 
 As you can see from the preceding listing:
 
-* We have enabled `Sink` binding (input-no-output) by using `@EnableBinding(Sink.class)`.
-Doing so signals to the framework to initiate binding to the messaging middleware, where it automatically creates the destination (that is, queue, topic, and others) that are bound to the `Sink.INPUT` channel.
-* We have added a `handler` method to receive incoming messages of type `Person`.
-Doing so lets you see one of the core features of the framework: It tries to automatically convert incoming message payloads to type `Person`.
+* We are using functional programming model (see <<Spring Cloud Function support>>) to define a single message handler as `Consumer`.
+* We are relying on framework conventions to bind such handler to the input destination binding exposed by the binder.
+
+Doing so also lets you see one of the core features of the framework: It tries to automatically convert incoming message payloads to type `Person`.
 
 You now have a fully functional Spring Cloud Stream application that does listens for messages.
 From here, for simplicity, we assume you selected RabbitMQ in <<spring-cloud-stream-preface-creating-sample-application,step one>>.
@@ -152,32 +153,18 @@ You can also build and package your application into a boot jar (by using `./mvn
 
 Now you have a working (albeit very basic) Spring Cloud Stream application.
 
-== What's New in 2.2?
-Spring Cloud Stream introduces a number of new features, enhancements, and changes in addition to the once already introduced in
-https://docs.spring.io/spring-cloud-stream/docs/Elmhurst.SR2/reference/htmlsingle/#_what_s_new_in_2_0[version 2.0]
-
-
-The following sections outline the most notable ones:
-
-* <<spring-cloud-stream-preface-new-features>>
-* <<spring-cloud-stream-preface-notable-enhancements>>
+== What's New in 3.0?
+TBD
 
 [[spring-cloud-stream-preface-new-features]]
 === New Features and Components
+TBD
 
 
 [[spring-cloud-stream-preface-notable-enhancements]]
 === Notable Enhancements
-
+TBD
 
 [[spring-cloud-stream-preface-notable-deprecations]]
 === Notable Deprecations
-
-As of version 2.2, the following items have been deprecated:
-
-- The spring-cloud-stream-reactive module is deprecated in favor of native support
-  via <<spring_cloud_function, Spring Cloud Function>> programming model.
-
-=== Notes on migrating from 1.x to 2.x?
-- Due to the improvements in content-type negotiation, the `originalContentType` header is not used (ignored) since 2.x and only exists for maintaining compatibility with 1.x versions
-- Introduction of `@StreamRetryTemplate` qualifier. While configuring custom instance of the `RetryTemplate` and to avoid conflicts you must qualify the instance of such `RetryTemplate` with this qualifier. See <<Retry Template, Retry Template>> for more details.
+TBD