perf: Add dedicated collectFirst implementation.

[He-Pin]

Motivation:
Use dedicated collectFirst implementation

Result:
Better performance.

[pjfanning]

This new class looks a lot like existing Akka based code. Is it truly new? Otherwise, use the Lightbend and Apache header from the original file.

[He-Pin]

Yes it's totally new, I implemented with not looking at any code of the origin Collect and this mechanism is generic and not diverged from lightbend's code, otherwise, every class that extends the GraphStage will add the Lightbend's license header? why should I? I think every collectFirst will implemented this way.

And you will see this kind of code in typelevel/fs2#3379 too and I chated at https://contributors.scala-lang.org/t/can-scala-compiler-do-the-micro-optimization-for-simple-pattern-matching/6493

[pjfanning]

lgtm

[mdedetrich]

lgtm

@pjfanning I can also confirm that if you wanted to write this from scratch it would also look like this. While it may look similar to existing akka code, there isn't much you can do about that because generally all of this code has the same style so if you want to write new functionality and keep the existing style it has a very high chance of looking this way.

[He-Pin]

Yes, we can ask anyone to write this code, and anyone's code will result in this style.