[foreign-memaccess] RFR 8225515: Remove MemoryScope from public API

[Maurizio Cimadamore]

Hi,
this is a followup from the discussion in [1]. Essentially, having a 
MemoryScope abstraction seems a bit heavy given the use we would make of 
it in the low level access API.

The discussion sparked very useful discussions on the role of 
confinement, which cannot be optional (as initially proposed): doing so 
could result in rogue threads to access already freed memory, thus 
resulting in a VM crash. Following that discussion, confinement is now 
enforced in all memory accesses, so that we have a clean and safe API.

Of course, the confined model could be too restrictive in certain cases, 
I'm working on a writeup to explain the problem, and possible solutions 
- stay tuned.

Here's the webrev:

http://cr.openjdk.java.net/~mcimadamore/panama/8225515/

Note: the webrev includes a patch (thanks Vlad!) to special case Thread 
constants in the JIT (otherwise we were seeing repeated calls to 
Thread.currentThread() in the profile). Performances of this patch are 
really good, and only few percents away from basic Unsafe access. There 
is, we believe, some more work to be done when it comes to array-like 
access - in which some of the range check eliminations (RCE) implemented 
by the JIT fail because we're using long indices rather than int (which 
are normally used as array indices in Java). We're working on that too.

Cheers
Maurizio

[1] - 
https://mail.openjdk.java.net/pipermail/panama-dev/2019-May/005644.html