RFR: 8342693: Use byte[] as parameter in a FDBigInteger constructor and as field

[Raffaello Giulietti]

This helps in reducing memory consumption, especially for long inputs.

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Commit messages:
 - Adjusted test to make use of byte[].
 - 8342693: Use byte[] as parameter in a FDBigInteger constructor and as field

Changes: https://git.openjdk.org/jdk/pull/21614/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21614&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342693
  Stats: 13 lines in 3 files changed: 0 ins; 0 del; 13 mod
  Patch: https://git.openjdk.org/jdk/pull/21614.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21614/head:pull/21614

PR: https://git.openjdk.org/jdk/pull/21614