[foreign-memaccess] RFR: 8254343: Revisit API for supporting mapped memory segments (second option)

[Maurizio Cimadamore]

This patch contains an alternate approach to support mapped memory segment operations,
effectively addressing the issues described in PR/377.

Here, support for mapped segments is reduced to bare minimum in the public API; a mapped segment is a segment whose
file descriptor is set.

A separate helper class `MappedMemorySegments` exposes the same routines which are now provided by
`MappedMemorySegment`.

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Commit messages:
 - Fix whitespaces
 - Alternate approach to support mapped memory segment operations
 - Address review comments.
 - Revamp API for mapped segments
 - Add scope checks to MappedMemorySegment operations

Changes: https://git.openjdk.java.net/panama-foreign/pull/378/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=378&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254343
  Stats: 534 lines in 13 files changed: 321 ins; 157 del; 56 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/378.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/378/head:pull/378

PR: https://git.openjdk.java.net/panama-foreign/pull/378