[foreign-memaccess+abi] RFR: Refactor compound layouts such as SequenceLayout and StructLayout

[Per Minborg]

This PR proposes harmonizations between `SequenceLayout` and `GroupLayout` as well as some improvements to the API exposing fewer details.

In this PR, there is an extra level that unifies and deduplicates the common methods in Sequence and Group Layouts. This level is visible in the sealed hierarchy. It would be possible to make it a separate interface that is not in the sealed hierarchy as an alternative.

![graphviz (23)](https://user-images.githubusercontent.com/7457876/189898114-b1fc8a44-7ce0-43ad-a82e-d62b3ce0e110.svg)

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Commit messages:
 - Move SequenceLayout from GroupLayout and add CompoundLayout
 - Move SequenceLayout to GroupLayout

Changes: https://git.openjdk.org/panama-foreign/pull/719/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=719&range=00
  Stats: 693 lines in 23 files changed: 380 ins; 252 del; 61 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/719.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/719/head:pull/719

PR: https://git.openjdk.org/panama-foreign/pull/719