[foreign] RFR 8210264: cleanup semantics of function pointer conversion

[Maurizio Cimadamore]

Hi,
this patch changes the way in which we support callbacks - for more 
details on the rationale, please see the JBS issue:

https://bugs.openjdk.java.net/browse/JDK-8210264

The idea is to model all callback with instances of the type:

Callback<T> (where T must be a functional interface)

and then have a method in Scope to do the conversion:

Callback<T> allocateCallback(T t)

This makes our 'raw' binding a lot more regular (and... well, raw). 
Callbacks can be passed onto native function by simply extracting their 
underlying pointer. All the magic is in the Callback::asFunction method 
which maps a native callback into a Java functional interface instance 
which can be used for the invocation.

Unsurprisingly, this fix has a cascade effect of simplifications in the 
binder backend - see NativeInvoker and UpcallHandler. Some classes such 
as UpcallHandlerFactory and UpcallStub were no longer needed and have 
been removed.

Now a callback will stay alive until the scope in which it has been 
created is alive (e.g. its close() method has not been called). When 
that happens the callback is freed. This means no more monkey patching 
with a Cleaner object and no more need for GC barriers to prevent early 
deallocation.

Webrev:

http://cr.openjdk.java.net/~mcimadamore/panama/8210264/

Maurizio