implement jshell support

[rogerkeays]

$ jshell -C-Xplugin:fluent
An exception has occurred in the compiler (17.0.2). Please file a bug against the Java compiler via the Java bug reporting page (http://bugreport.java.com) after checking the Bug Database (http://bugs.java.com) for duplicates. Include your program, the following diagnostic, and the parameters passed to the Java compiler in your report. Thank you.
java.lang.NoClassDefFoundError: sun/misc/Unsafe
	at com.sun.tools.javac.comp.Fluent.init(Fluent.java:26)
	at jdk.compiler/com.sun.tools.javac.api.BasicJavacTask.initPlugin(BasicJavacTask.java:255)
	at jdk.compiler/com.sun.tools.javac.api.BasicJavacTask.initPlugins(BasicJavacTask.java:229)
        ...

We need to replace Unsafe to be future proof anyway.

Unsafe source: https://github.com/openjdk/jdk/blob/master/src/jdk.unsupported/share/classes/sun/misc/Unsafe.java