[foreign] RFR 8217245:Unify functions and layouts

[Maurizio Cimadamore]

Hi,
this is a followup for the RFC here [1]. The goal is to put both Layout 
and Function under a common root, called Descriptor (this name is very 
broad so we will change it at a later stage).

I've put together a new webrev which addresses some of the comments 
raised in [1]. This is an official request for review.

Webrev:

http://cr.openjdk.java.net/~mcimadamore/panama/layout%2bfunc_v3/

Cheers
Maurizio

[1] - 
https://mail.openjdk.java.net/pipermail/panama-dev/2018-November/003279.html