[foreign-memaccess+abi] RFR: 8300785: Add way to select group element paths by index

[Maurizio Cimadamore]

This patch adds a new factory to MemoryLayout.PathElement, namely, to create a group element path from a long index, rather than a name. While named struct/union fields are common, there are cases where being able to access elements by index is important, such when doing a full (recursive) scan of the contents of a struct.

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/panama-foreign/pull/773/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=773&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300785
  Stats: 85 lines in 3 files changed: 57 ins; 0 del; 28 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/773.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/773/head:pull/773

PR: https://git.openjdk.org/panama-foreign/pull/773