RFR: 8300235: Use VarHandle access in Image(Input | Output)Impl classes

[Per Minborg]

This PR suggests improving performance by using the newly introduced class `jdk.internal.util.ByteArray` to improve packing/unpacking operations.

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Commit messages:
 - Remove comments
 - Add little endian performance improvements
 - Use jdk.internal.util.ByteArray for pack/unpack

Changes: https://git.openjdk.org/jdk/pull/12204/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12204&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300235
  Stats: 279 lines in 5 files changed: 8 ins; 144 del; 127 mod
  Patch: https://git.openjdk.org/jdk/pull/12204.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12204/head:pull/12204

PR: https://git.openjdk.org/jdk/pull/12204