[foreign] annotation refactoring towards the spec

[Sundararajan Athijegannathan]

Refactoring annotations towards specification: 
http://cr.openjdk.java.net/~mcimadamore/panama/panama-binder-v3.html

* NativeCallback annotation for callback Functional interfaces.
* NativeStruct annotation for structs/union
* removed CallingConvention.java - which is unused in binder
* renamed "C" to be NativeLocation - to be consistent with other 
annotation names
* Binder picks up struct layout from NativeCallback, NativeStruct (and 
not from NativeType on functional interface method or struct interface)
* NativeHeader has "declarations" attribute that has all descriptions 
collected in one place.
* removed unused "size" attribute in NativeType.

Webrev: 
http://cr.openjdk.java.net/~sundar/annotation_refactoring/webrev.00/index.html

Thanks,
-Sundar