Implement a pure-Java version of AccessController.doPrivileged. This enhancement has several benefits. One, it should improve performance by avoiding calls into the JVM. Currently, applications that don't use a SecurityManager still have to incur the expense of doPrivileged. This enhancement should avoid much of that. Second, a pure-Java implementation of AccessController.doPrivileged will avoid having native frames on the stack, which is desirable for Project Loom (https://openjdk.java.net/projects/loom/).