[foreign-memaccess+abi] RFR: Refactor compound layouts such as SequenceLayout and StructLayout [v2]

[Per Minborg]

On Tue, 13 Sep 2022 12:30:33 GMT, Per Minborg <duke at openjdk.org> wrote:

>> 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)
>
> Per Minborg has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Revert name of GroupLayout implementation

We decided not to go down this route so I am closing this PR.

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