[foreign-abi] RFR: Clean up Binding

[Jorn Vernee]

Hi,

Please review the following patch that does a few cleanups of the 
Binding class implementation:
- Adds javadoc
- Uses static factory methods instead of raw constructors. This allows 
returning singletons for some of the operators for instance.
- Constructing a COPY_BUFFER binding now requires a MemoryLayout, 
instead of a separate size & alignment. This was easier given the use-cases.
- Binding.AllocateBuffer renamed to Binding.Allocate (since we already 
had Copy as well).
- BoxAddress renamed to ConvertAddress, as a more general name to 
describe the boxing & unboxing that can occur. Suggested by Maurizio.

Webrev: 
http://cr.openjdk.java.net/~jvernee/panama/webrevs/cleanup_bindings/webrev.00/
Bugs: https://bugs.openjdk.java.net/browse/JDK-8237359, 
https://bugs.openjdk.java.net/browse/JDK-8238227, 
https://bugs.openjdk.java.net/browse/JDK-8238235

This patch take care of several smaller issues all at once. It applies 
on top of the JDK-8237360 patch.

Thanks,
Jorn