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

[Per Minborg]

This PR moves some of the internal concerns from `MemorySegment` to the internal `AbstractMemorySegmentImpl`.

Also, some renaming of static fields.

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Commit messages:
 - Fix imports
 - Break out functionalty to internal class

Changes: https://git.openjdk.org/panama-foreign/pull/727/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=727&range=00
  Stats: 321 lines in 8 files changed: 156 ins; 125 del; 40 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/727.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/727/head:pull/727

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