[foreign-memaccess] RFR: 8253243: Investigate ways to make MemorySegment::ofNativeRestricted more composable

[Maurizio Cimadamore]

This patch simplifies the MemorySegment::ofNativeRestricted API, based on a number of observations:

* now that we have shared segment support, passing an explicit owner thread isn't that useful (e.g. this is no longer an
  escape hatch, and can - and should - be done with public, safe API)
* if we had a public safe API to add cleanup action to existing segments, we could also get rid of the cleanup parameter
* if a custom cleanup action embedded some other object, that object will be kept reachable, so no attachment API is
  needed
* if we moved the method to MemoryAddress, we would streamline uses of this API quite a bit, as in
  `address.asSegmentRestricted(100)` - e.g. only size is needed

Adding the new API for attaching cleanup action is relatively straightforward - as usual we have to dup the segment and
the scope, and create a new segment, with a new scope whose cleanup action calls the custom runnable before the
original action (that original action should still be called, since otherwise we would fail to free/unmap memory). Any
exception thrown by the cleanup action is caught and ignored, so as not to mess with the critical cleanup which has to
occur.

A nice consequence of this work is that the surface for unsafety is much reduced, and the various methods compose much
better. For instance, the "everything" segment can be expressed as follows:

MemoryAddress.NULL
    .asSegmentRestricted(Long.MAX_VALUE)
    .withOwnerThread(null)
    .withAccessModes(READ | WRITE);

Which is, I think, much nicer. The changes in tests also suggest that the new API is easier on clients, and in one case
(TestCleaner) the need for opting into unsafe operation actually vanished.

I also did a bit of cleaning on the javadoc, which appeared to be off-sync when it comes to confinement.

-------------

Commit messages:
 - Ignore exceptions thrown by the custom cleanup action
 - Initial work

Changes: https://git.openjdk.java.net/panama-foreign/pull/328/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=328&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253243
  Stats: 216 lines in 11 files changed: 94 ins; 66 del; 56 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/328.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/328/head:pull/328

PR: https://git.openjdk.java.net/panama-foreign/pull/328