[foreign-memaccess+abi] RFR: Micro optimizations and tidy

[Per Minborg]

This small PR adds some optimizations and tidies up some imports.

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Commit messages:
 - Micro optimizations and tidy

Changes: https://git.openjdk.org/panama-foreign/pull/708/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=708&range=00
  Stats: 32 lines in 4 files changed: 7 ins; 15 del; 10 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/708.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/708/head:pull/708

PR: https://git.openjdk.org/panama-foreign/pull/708