[foreign-memaccess+abi] RFR: Rename layout variable and parameter names to be consistent

[Per Minborg]

This PR fixes naming inconsistencies across various implementations of `MemoryLayout`.

New users are likely to understand classes more easily when there is a consistent naming strategy.

-------------

Commit messages:
 - Rename layout variable and parameter names to be consistent

Changes: https://git.openjdk.org/panama-foreign/pull/706/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=706&range=00
  Stats: 118 lines in 5 files changed: 0 ins; 0 del; 118 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/706.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/706/head:pull/706

PR: https://git.openjdk.org/panama-foreign/pull/706