RFR: 8310813: Simplify and modernize equals, hashCode, and compareTo for BigInteger

[Pavel Rappo]

Please review this PR to use modern APIs and language features to simplify equals, hashCode, and compareTo for BigInteger. If you have any performance concerns, please raise them.

This PR is cherry-picked from a bigger, not-yet-published PR, to test the waters. That latter PR will be published soon.

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Commit messages:
 - Improve
 - Initial commit

Changes: https://git.openjdk.org/jdk/pull/14630/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14630&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310813
  Stats: 30 lines in 1 file changed: 3 ins; 20 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/14630.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14630/head:pull/14630

PR: https://git.openjdk.org/jdk/pull/14630