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

[Per Minborg]

This PR brakes out functionality from `MemorySegment` into an internal class.

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Commit messages:
 - Break out functionality to internal class

Changes: https://git.openjdk.org/panama-foreign/pull/732/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=732&range=00
  Stats: 274 lines in 2 files changed: 152 ins; 109 del; 13 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/732.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/732/head:pull/732

PR: https://git.openjdk.org/panama-foreign/pull/732