[foreign-memaccess] RFR 8224039: Remove unnecessary layout classes

[Maurizio Cimadamore]

Hi,
this patch removes layout subclasses which were deemed unnecessary for 
this memory access API:

* Address (this API is about contiguous memory access)
* Unresolved (by name references is especially useful in combination 
with pointers)

This patch also removes the Value::contents method, on the basis that 
for now there's nothing sensible the API can do with it (this will 
change once we can rely upon the Vector API).

Note: the removed layouts are not gone forever, and will likely be 
re-added by higher level layers.

Webrev:

http://cr.openjdk.java.net/~mcimadamore/panama/8224039/

Cheers
Maurizio