[foreign] RFR: Add support for layout resolution

[Maurizio Cimadamore]

Hi,
during last week changes, I've left behind some parts related to layout 
symbolic name resolution (e.g. Unresolved layout).

This patch adds that; support is very simple, e.g. an Unresolved layout 
is resolved by looking at its 'name' annotation, and looking that up 
inside a shared map/dictionary held by the binder.

This allows to simplify quite a few tests.

I also had to make few tweaks to the ABI classes to be able to handle 
unresolved layouts.

Webrev:
http://cr.openjdk.java.net/~mcimadamore/panama/layout_resolver/


Cheers
Maurizio