RFR: implementation for JEP 334: JVM Constants API

[jbvernee]

Hello (again),

Sorry for keeping this going, but I keep thinking of new things a few 
hours later (there goes my Sunday :/).

I'm wondering if the return type of `Constable::describeConstable` is 
actually correct. Right now it's `Optional<? extends ConstantDesc<? 
super T>>`, but why are you using `ConstantDesc<? super T>`? Since 
`ConstantDesc` is a _producer_ of `T` PECS dictates that it should use 
co-variance, i.e.: `ConstantDesc<? extends T>`, so that would make 
`Optional<? extends ConstantDesc<? extends T>>` the return type. (a 
ConstantDesc that resolves to a String can also be viewed as a 
ConstantDesc that resolves to an Object)

That way you don't even have the problem I was trying to solve before, 
and the return type of `EnumDesc::describeConstable` (and similarly for 
`VarHandleDesc`) can be `Optional<DynamicConstantDesc<EnumDesc<E>>>` 
which is the exact type that is being returned, which is what is wanted 
right?

On that note, you could also change the return type of 
`describeConstable` for the box types and String to return an optional 
of the type itself (since they're their own descriptor, each implements 
`ConstantDesc` of itself). For instance, for `Double` this would change 
from `Optional<ConstantDesc<Double>>` to `Optional<Double>`.

Best regards,
Jorn Vernee