[foreign-memacces] RFR 8227278: Remove CompoundLayout

[Maurizio Cimadamore]

Hi,
this is an official RFR for the work described here:

https://mail.openjdk.java.net/pipermail/panama-dev/2019-July/005906.html

I've made few changes to the patches shared there - more specifically:

* I removed the Layout::elements method
* I removed GroupLayout::elementAt and GroupLayout::elementsSize, and 
iterable interface - and replaced them with GroupLayout::memberLayouts() 
-> List<Layout> - after all, once you have the list of member layouts 
you can obtain all the other info as well
* I tightened the wording: a group layout has _member_ layouts; a 
sequence layout has an _element_ layout.

The rest should be the same as yesterday.

Webrev:

http://cr.openjdk.java.net/~mcimadamore/panama/8227278/

javadoc:

http://cr.openjdk.java.net/~mcimadamore/panama/8227278_javadoc

Cheers
Maurizio