[foreign-memaccess+abi] RFR: Deduplicate layouts and simplify value layout implementations [v3]

[Per Minborg]

On Thu, 16 Feb 2023 09:44:19 GMT, Per Minborg <pminborg at openjdk.org> wrote:

>> This PR proposes adding deduplication of the immutable value layout where one provides a value that is equal to the one already at hand. For example, `JAVA_INT.withBitAlignment(32)` will return the same object. There is no general caching mechanism that guarantees a layout will never repeat. 
>> 
>> Another part of this PR relates to simplifying value layouts so they only have a single constructor. This will reduce size slightly. Related to this is the replacement of certain magic numbers which are now represented by constants.
>
> Per Minborg has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Remove dedup for names and clean up

I am closing this PR now. We could always come back if duplicate instances becomes a problem. Thanks for the feedback on this PR.

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PR: https://git.openjdk.org/panama-foreign/pull/795