[foreign-preview] RFR: 8281855: Rename ResourceScope to MemorySession

[Maurizio Cimadamore]

This patch implements some of the changes described in [1] and [2]. More specifically, the `ResourceScope` abstraction is renamed into `MemorySession`, and the session accessors in `MemorySegment`, `NativeSymbol` and `VaList` are tweaked to return *non-closeable* session views. To counteract this change, `MemorySession` now supports `equals` and `hashCode`, so that sessions views can be compared to each other in ways that are not identity-sensitive.

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Commit messages:
 - Fix whitespaces
 - Merge branch 'foreign-preview' into session_preview
 - Fix reference to ResourceScope in javadoc
 - Fix MemorySession javadoc
 - Initial push

Changes: https://git.openjdk.java.net/panama-foreign/pull/641/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=641&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8281855
  Stats: 4685 lines in 136 files changed: 1501 ins; 1333 del; 1851 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/641.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/641/head:pull/641

PR: https://git.openjdk.java.net/panama-foreign/pull/641