[foreign-memaccess+abi] RFR: ValueLayouts: Improve test coverage and more [v2]

[Jorn Vernee]

On Mon, 13 Feb 2023 10:21:11 GMT, Per Minborg <pminborg at openjdk.org> wrote:

>> This PR:
>> 
>> * Adds tests to `ValueLayout` methods
>> * Simplifies an `equals()` method
>> * Changes from exception throwing to assert for parameters that cannot be changed client-side
>
> Per Minborg has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Update after comments

Marked as reviewed by jvernee (Committer).

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PR: https://git.openjdk.org/panama-foreign/pull/786