[foreign-memaccess+abi] RFR: 8296315: Add get/set-AtIndex methods for byte, boolean

[Per Minborg]

This PR proposes adding four methods for getting and setting `byte` and `boolean` values via index.

It also contains a drive-by fix of a test where the cases for `short` was missing.

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Commit messages:
 - Add get/set-AtIndex methods for byte, boolean

Changes: https://git.openjdk.org/panama-foreign/pull/818/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=818&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296315
  Stats: 116 lines in 2 files changed: 115 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/818.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/818/head:pull/818

PR: https://git.openjdk.org/panama-foreign/pull/818