Add support for Java 9 String concatenation

[CoderBaron]

This PR adds support for Java 9 String concatenation by rewriting its invokedynamic instructions to StringBuilder appends.

This makes it possible to compile your Java 9+ code with RoboVM. New language features like private/static/default interface methods [Java 9], local-variable type inference (the var keyword) [Java 10], switch expressions [Java 14] or text blocks [Java 15] will work now.

Caution when using Java 11+: Nested classes are handled differently from now on. Previously you have been able to access private methods/fields of nested classes in the outer class and vice versa, however this will now result in an IllegalAccessError.

[colorizenl]

@CoderBaron Thanks for this, should be extremely useful. Until now it was possible to use RoboVM for JAR files compiled with Java 9-11, but it requires the following Gradle option:

compileJava {
    options.compilerArgs << "-XDstringConcat=inline"
}

Obviously, this only works for your own code. Any libraries compiled with Java 9+ would become unusable, which should be fixed by this. Obviously, the libraries would still not work if they use new APIs, so in parallel to this I'm also eagerly following #527.

[dkimitsa]

@CoderBaron
thanks looks good, could you please share more information about "java 11+ Nested classes are handled differently" like JEP.
Thx !

[CoderBaron]

In JEP 181 they added support for accessing private nested members directly, dropping the need to create access bridge methods. Of course with a runtime below version 11, Java fails to find these methods.