[foreign-memaccess] RFR: 8254343: Revisit API for supporting mapped memory segments [v2]

[Maurizio Cimadamore]

> As I was polishing the memory access API ahead of the upstream integration, I realized of a fatal flaw with the
> `MemorySegment` vs. `MappedMemorySegment` split. This split works against the idiomatic usage of the VarHandle API:
> since memory access var handles feature a `MemorySegment` coordinate, clients using a `MappedMemorySegment` will be
> using *inexact* access mode (which goes through an extra, expensive `asType` adaptation).   The only way to remove the
> perfomance issue is to make deep changes in the VarHandle machinery to support a more lax form of `asType` adaptation,
> but adding this to support mapped segments seems, frankly, overkill.  On top of that, while discussing this with Paul,
> I realized that no liveness check was performed on the load()/unload() operation, and that these operations, more
> generally, did not get the `@Scoped` treatment, meaning they will crash the VM when  used with shared segments.  This
> patch rectifies these issues; it removes the `MappedMemorySegment` interfaces, and it introduce a `MemoryMapping`
> abstraction instead. Clients can ask a memory mapping out of a segment; the memory mapping will contain the usual
> `load`/`force` operation, so there's no loss in expressiveness of the API.  For instance, to force contents of a mapped
> segment to be written in the underlying file, a client can do:  segment.asSlice(20, 100)
>        .mapping()
>        .ifPresent(MemoryMapping::force);
> 
> We believe this to be a reasonable compromise, which allows us to keep a single MemorySegment interface (instead of
> two), which removes the var handle performance issue, and in turn enables other improvements, such as turning the
> `spliterator` method into a true instance method.

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

  Address review comments.
  Apply some renaming: `mapping` -> `toMemoryMapping`.
  Also streamlined javadoc a bit.

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

Changes:
  - all: https://git.openjdk.java.net/panama-foreign/pull/377/files
  - new: https://git.openjdk.java.net/panama-foreign/pull/377/files/18c63bbc..6c795775

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=377&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=377&range=00-01

  Stats: 39 lines in 6 files changed: 0 ins; 0 del; 39 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/377.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/377/head:pull/377

PR: https://git.openjdk.java.net/panama-foreign/pull/377