[foreign-memaccess+abi] RFR: 8291473: Unify MemorySegment and MemoryAddress

[Maurizio Cimadamore]

This patch implements the changes described in [1].

The main things to notice are the removal of `MemoryAddress` and `Addressable`. Everything else falls from that.

There are some changes to the `MemorySegment` API as well, as some methods such as `segmentOffset` and `asOverlappingSlice` have been removed, and instead `MemorySegment::address` now works on heap segments too (and, clients can obtain the array of an heap segment using its `array` method). So, these methods can, if needed, be implemented outside the core API.

`MemorySegment::equals` now compares the segments address, which is the most useful comparison when working with native interop (again, client can implement deeper comparison using `mismatch`).

Finally, restricted factories accepting raw addresses (in `MemorySegment` and `VaList`) have been tweaked to accept a long value instead.

`VaList` also needed some adjustments, since it builds on top of segments. The API is similar to before - but instead of having an address accessor, it has a zero-length segment accessor.

`ValueLayout.OfAddress` has a new method to create *unsafe* address layouts which return segment with maximal size (`Long.MAX_VALUE`). I've used these layouts in a lot of places in the implementation internal, which simplifies things quite a bit (e.g. removing the need to create a new segment from an address).

There are several test and microbenchmark updates, but relatively minor, and all caused by removal of `MemoryAddress`/`Addressable`. Changes here should be relatively simple to follow.

While the javadoc is ok, I didn't put too much effort in trying to provide a complete and cohesive story on how zero-length segments are used to model raw addresses. I'm working on some more extensive doc changes on the side, but I wanted to split the javadoc changes from the impl changes, so as to simplify the review work. I hope that makes sense.

[1] - https://mail.openjdk.org/pipermail/panama-dev/2022-July/017181.html

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Commit messages:
 - Tweak javadoc for MemorySegment::get(Address)
 - Merge branch 'foreign-memaccess+abi' into ffm_segment_unification
 - Simplify TestUpcallHighArity
 - Simplify TestUpcallBase
 - Further minimize diffs
 - Minimize diffs
 - Initial push

Changes: https://git.openjdk.org/panama-foreign/pull/694/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=694&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8291473
  Stats: 3799 lines in 116 files changed: 644 ins; 2312 del; 843 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/694.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/694/head:pull/694

PR: https://git.openjdk.org/panama-foreign/pull/694