Upgrade to ASM 6.1

    new V10 constant for Java 10 class files
    xml package deprecated
    project layout changed to Maven style
    build system changed from Ant to Gradle
    coding style changed to Google Java style
    code quality and readability improved a lot
    code quality tracked with SonarQube
    performance improvements
    unit tests improved: migrated to JUnit 5, faster, coverage > 95%
    non-regression tests added for past issues
    benchmarks improved: now use JMH
    new Gitlab Continuous Integration pipeline
    bug fixes:
        317800: Resizing jump instruction adds invalid stack frame to a class without frames
        317802: MethodRemapper.visitInvokeDynamicInsn mutates source bsmArgs
        317804: Adjust CheckModuleAdapter so Module attribute in 54.0+ class file cannot contains a requires java.base with ACC_TRANSITIVE or ACC_STATIC_PHASE
        317807: Asymmetry of ClassWriter for bytecode versions higher than the one that ClassReader accepts
        317809: CheckMethodAdapter rejects valid Java and JVM identifiers with surrogate characters
        small bug fixes related to modules (https://gitlab.ow2.org/asm/asm/merge_requests/20/, https://gitlab.ow2.org/asm/asm/merge_requests/19/)
    backward binary compatibility is preserved, but the behavior at runtime has changed in a few areas:
        ACC_SYNTHETIC flag vs Synthetic attribute, see https://gitlab.ow2.org/asm/asm/merge_requests/17
        numbering of synthetic parameters: see https://gitlab.ow2.org/asm/asm/merge_requests/56
        StringBuffer no longer used in util (Printer.buf deprecated, use stringBuilder instead)