Make blaze native-image compatible

[asm0dey]

Corresponding issue in graalvm tracker: oracle/graal#772
Main point: currently blaze uses nashorn as js engine, potentially using graal.js instead will make blaze native-image build-able.
If it will be possible to build blaze with native-image then we'll get almost imstant startup time, which is must have for scripting.

[agentgt]

I just glanced through blazes code base and even if nashhorn was removed/replaced it still would not work for plain Java because Blaze uses dynamic reflection to call the methods as seen here:

https://github.com/fizzed/blaze/blob/master/blaze-core/src/main/java/com/fizzed/blaze/jdk/TargetObjectScript.java

One of the problem is that there isn't really a truffle implementation of Java. Consequently the "Script" would have to be compiled either with just the regular JDK or JDK + native-image. native-image blaze.jar calling slower JDK compiled Java is probably slower than just a pure JDK (just a guess).

The only way I can see this working blaze would have to do code generation to build a stub to call the tasks to avoid the reflection and then call the graalvm to compile the Java code to native.