While nestmates will ultimately be associated with a future source/target version of the JDK and an updated classfile version (see JDK-8187302) we want to be able to enable them by default as-if executing in that future release.

The primary changes are confined to javac and are very straight-forward:
- generate the nestmate attributes in the classfile
- use virtual invocation bytecodes for private method invocations
- don't generate private accessors

With this change in place we can update the hotspot nestmate tests to not provide explicit "@compile" directives.