[foreign] RFR 8210998: Missing pointer scope checks

[Maurizio Cimadamore]

Hi,
while playing around with the Panama API in jshell, I realized that we 
don't always enforce scope liveness in all contexts. One of the most 
important cases was:

Pointer::addr

But there were other cases left out too (see Reference subclasses).

This patch adds the missing checks in the right places and also add a 
comprehensive test which check:

* Pointer dereference (get/set) after Scope::close() with various 
pointer shapes

* Struct/array/callback access after Scope::close

* Passing pointer/struct/callback after Scope::close

In writing this tests I uncovered few issues:

1) The signature of LayoutType.ofFunction was wrong - it had a 
Callback<Z> bound on the type variable which was bogus. In fact there's 
no bound here (this is a leftover from previous code). This also 
required tweaks to LayoutTypeImpl

2) Scope::allocateCallback was NOT associating the callback entry point 
pointer with the current scope. As a result the callback pointer was 
accessible even AFTER the owning scope was closed. I fixed it by moving 
the pointer creation logic inside Scope - now UpcallHandler just has a 
'long' entry point, which I think makes the code even clearer.

3) CallbackImplGenerator - this code was going out of his way in order 
to enforce checks on pointer scope; however, on a closer inspection, 
since this code relied on 'addr()' and Pointer::addr is now checked 
properly, there's no need to add special cruft.

4) The logic for 'setting' arrays into array references was overly 
convoluted: with Array we can now just do the operation with a bulk 
pointer copy.

Webrev:

http://cr.openjdk.java.net/~mcimadamore/panama/8210998/

Cheers
Maurizio