[foreign] RFR: native libraries should have a global scope

[Maurizio Cimadamore]

Hi,
In general, all user-facing pointers have a scope; there is however an 
important exceptions: pointer to globals. Since such pointers are 
computed inside the binder, they do not have any associate scope. This 
leads to issues with code uniformity (where e.g. we have to defensively 
check for null when retrieving a pointer's scope), and it is also an 
impediment to develop certain features: for instance, we'd like constant 
values to be generated in the binder - and not in jextract (as now) - 
but doing so might require native memory allocation by the native 
library (think about a string constant) - and that requires a scope.

This patch adds a global scope that is shared by all instance of a 
certain library. The scope is created when an impl for a library 
interface is requested. A reference cleaner is also used to make sure 
that the Scope::close method is called when the library class is unloaded.

As part of this change I also added javadoc to the Library interface, 
and turned the Library.Symbol class into an interface (allocating a 
symbol is a privileged operation and should not be exposed by the public 
API).

Webrev:

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


Maurizio