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

[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.

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

Commit messages:
 - Revamp API for mapped segments
 - Add scope checks to MappedMemorySegment operations

Changes: https://git.openjdk.java.net/panama-foreign/pull/377/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=377&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254343
  Stats: 501 lines in 10 files changed: 295 ins; 154 del; 52 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