[foreign-memaccess+abi] RFR: 8301801: Implement arena-centric API [v8]

[Maurizio Cimadamore]

> This patch implements the API proposal described here:
> 
> https://cr.openjdk.java.net/~mcimadamore/panama/scoped_arenas.html
> 
> The main changes are:
> 
> * Static factories `MemorySegment::allocateNative` are gone;
> * The static factory `SegmentAllocator::nativeAllocator` is also gone;
> * `SegmentScope` moved as nested class in `MemorySegment` (e.g. `MemorySegment.Scope`);
> * Tweak methods accepting `SegmentScope` to accept `Arena` instead;
> * Simplify `Arena` API, by removing predicates (`isCloseable`/`isAccessibleBy`) and by dropping `whileAlive`;
> * Change arena factories to use `of` prefix instead of `open` - and rename `auto()` to `ofAuto()`.
> 
>  The API changes are rather straightforward, but they lead to several shallow test and microbenchmark changes, so the number of affected file is quite big (mostly because of the rename `openConfined` -> `ofConfined`).
> 
> A javadoc of the proposed changes is available here:
> 
> http://cr.openjdk.java.net/~mcimadamore/panama/scoped_arena_interface_lump_predicates/

Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:

  Fix tests outside foreign package

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

Changes:
  - all: https://git.openjdk.org/panama-foreign/pull/781/files
  - new: https://git.openjdk.org/panama-foreign/pull/781/files/089009cc..50c7e712

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=781&range=07
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=781&range=06-07

  Stats: 296 lines in 38 files changed: 2 ins; 3 del; 291 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/781.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/781/head:pull/781

PR: https://git.openjdk.org/panama-foreign/pull/781