[foreign] RFR 8219230: Struct.toString implementation is outdated

[Jorn Vernee]

Hi,

After moving to method level @NativeGetter annotations the 
implementation of Struct.toString was not updated for the new scheme. 
I've update the implementation, and added a test. I reused the 
Struct.OffsetOf test class and renamed it to StructTest. I've also added 
as Struct.sizeOf test to the same class for good measure.

Bug: https://bugs.openjdk.java.net/browse/JDK-8219230
Webrev: 
http://cr.openjdk.java.net/~jvernee/panama/webrevs/8219230/webrev.00/

Thanks,
Jorn