Enable native image

[koppor]

This PR enables GraalVM Native Image. GraalVM Native Image allows yahead-of-time compile Java code to a standalone executable, called a native image.

As "side-effect", Gradle is updated to 5.4.1 and the dependency to picocli is fetched using gradle (instead of having the source copied into this repository).

I also started to enable CirceCI to have a "check in the cloud" whether everything compiles fine.

I will possibly swtich to Travis, because Travis offers building on MacOS and Windows. This might become necessary as GraalVM does not support cross compiliation (oracle/graal#407).

[adoble]

Thanks Oliver for the PR. One of the goals of the projects (ADR #2) was to have a tool that was OS independent, This seems to go a lot further than that so even better. I'm not familiar with GraalVM so give me some time to get up to steam before merging.

[koppor]

It is not required to use the native image. The "old" setup still works. That means, gradlew shadowJar still creates the fat jar containing all dependencies. This works independently of the used JDK.

I am using the project as test playground for GraalVM (to learn about the capabilities and difficulties). My second aim is that npm install adr-j will work (See adr#4). I know that there are projects including a JVM, but I want to try the native image, since the startup time is 0seconds instead of 1 to 2 seconds.

[adoble]

Understood. I've merged the PR.
Your future plan to use npm to replace the current manual install sounds good.