[foreign-memaccess+abi] RFR: Break out functionality to internal class

[Per Minborg]

On Mon, 19 Sep 2022 13:30:51 GMT, Per Minborg <duke at openjdk.org> wrote:

> This PR moves some of the internal concerns from `MemorySegment` to the internal `AbstractMemorySegmentImpl`.
> 
> Also, some renaming of static fields.

Superseded by https://github.com/openjdk/panama-foreign/pull/732

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PR: https://git.openjdk.org/panama-foreign/pull/727