JVMS changes for VarHandles

From the JEP 193 description:

The following additions will be required to the Java Virtual Machine Specification:

1. Make reference to the signature-polymorphic access mode methods in the VarHandle class.

2. Specify invokevirtual byte code behaviour of invocation to access mode signature-polymorphic methods. It is anticipated that such behaviour can be specified by defining a transformation from the access mode method invocation to a MethodHandle which is then invoked using invokeExact with the same parameters (see previous use of MethodHandles.Lookup.findVirtual). Further investigation is required to determine if refined transformations are possible (now, or in the future without breaking compatibility) to make access mode method similar to MethodHandle.invoke, for example, by transforming the method handle via MethodHandle.asType where all reference types are cast from Object.