RFR: implementation for JEP 334: JVM Constants API

[jbvernee]

> It's unfortunate that the return type of `describeConstable` in 
> `EnumDesc` and `VarhandleDesc` can't be simplified any more currently.

I might have gotten lost in my own train of thought a little bit there. 
I missed the fact that you can also change the return types to 
`Optional<ConstantDesc<ConstantDesc<E>>>` for `EnumDesc` and 
`Optional<ConstantDesc<ConstantDesc<Varhandle>>>` for `VarHandleDesc`, 
without changing anything else. Those are at least a little bit simpler.