[foreign-abi] RFR: JDK-8242061: AllocationScope is too general

[Maurizio Cimadamore]

This patch restricts allocation scope to work only on native allocations. This restriction allows us to lift the size
limitations from unbounded allocation scopes (as we can always fallback on a MemorySegment.allocateNative which
guarantees correct alignment).

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Commit messages:
 - Make AllocationScope native-only

Changes: https://git.openjdk.java.net/panama-foreign/pull/93/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/93/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-JDK-8242061
  Stats: 687 lines in 6 files changed: 258 ins; 361 del; 68 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/93.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/93/head:pull/93

PR: https://git.openjdk.java.net/panama-foreign/pull/93