[foreign] RFR: Make jextract Context immutable

[Jorn Vernee]

Hi,

This is an RFR for the earlier patch that makes Context immutable:
http://cr.openjdk.java.net/~jvernee/panama/webrevs/context/webrev.00/

To recap, this patch:

* Makes all fields of Context final, and wraps collections in immutable 
wrappers.

* Adds a Context.Builder to create an immutable Context.

* Rearranges code in Main to use the Context.Builder.

Thanks,
Jorn