[foreign] RFR 8219103: Add an offsetOf utility to find the offset of Struct fields

[Jorn Vernee]

Hi,

This is an RFR for the earlier proposed offsetOf utility for finding the 
offset of a field of a struct.

This is based on Maurizio's suggestion to use the LayoutPath API [1], 
with some minor tweaks and a test added.

Bug: https://bugs.openjdk.java.net/browse/JDK-8219103
Webrev: 
http://cr.openjdk.java.net/~jvernee/panama/webrevs/8219103/webrev.00/

Thanks,
Jorn

[1] : 
https://mail.openjdk.java.net/pipermail/panama-dev/2019-February/004360.html