[foreign] RFR 8211006: Add API points to support by-value semantics for structs and arrays

[Maurizio Cimadamore]

As written in the issue summary (see for more details):

https://bugs.openjdk.java.net/browse/JDK-8211006

We need API points to perform deep copy of structs/arrays, which would 
allow developers to easily mimic what happens when operating on 
struct/array struct fields.

I also did a minor tweak on References.ofArray, as the setter was 
redundantly creating an Array object instead of just copying.

Webrev:

http://cr.openjdk.java.net/~mcimadamore/panama/8211006/

Cheers
Maurizio