[foreign] RFR 8218669: Flatten Reference hierarchy

[Maurizio Cimadamore]

Hi,
this is a simple change which makes the Reference hierarchy in 
References.java flatter and 1-1 with respect to the carrier it models.

Some internal testing of linkToNative pointed at profile pollution 
because of multiple carriers using the same reference class (e.g. 
OfPrimitive).

This patch does 3 things:

1) Remove the intermediate AbstractReference class
2) Create one reference class per carrier, and make all getter/setter MH 
static constants
3) Consolidate the code my moving the Reference interface inside References.

Webrev:

http://cr.openjdk.java.net/~mcimadamore/panama/8218669/

Cheers
Maurizio