RFR: 8311864: Add ArraysSupport.hashCode(int[] a, fromIndex, length, initialValue) [v4]

[Pavel Rappo]

> This PR adds an internal method to calculate hash code from the provided integer array, offset and length into that array, and the initial hash code value.
> 
> Current options for calculating a hash code for int[] are inflexible. It's either ArraysSupport.vectorizedHashCode with an offset, length and initial value, or Arrays.hashCode with just an array.
> 
> For an arbitrary int[], unconditional vectorization might be unwarranted or puzzling. Unfortunately, it's the only hash code method that operates on an array subrange or accepts the initial hash code value.
> 
> A more convenient method could be added and then used, for example, here:
> 
> * https://github.com/openjdk/jdk/blob/0ef03f122866f010ebf50683097e9b92e41cdaad/src/java.base/share/classes/java/util/concurrent/CopyOnWriteArrayList.java#L1076-L1083
> 
> * https://github.com/openjdk/jdk/blob/0ef03f122866f010ebf50683097e9b92e41cdaad/src/java.base/share/classes/java/util/ArrayList.java#L669-L680
> 
> * https://github.com/openjdk/jdk/blob/0ef03f122866f010ebf50683097e9b92e41cdaad/src/java.base/share/classes/sun/security/pkcs10/PKCS10.java#L356-L362
> 
> This PR adds such a method and provides tests for it. Additionally, this PR adds tests for `null` passed to `java.util.Arrays.hashCode` overloads, behaviour which was specified but untested.

Pavel Rappo has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains four additional commits since the last revision:

 - Merge branch 'master' into 8311864
 - Merge remote-tracking branch 'jdk/master' into 8311864
 - Merge branch 'master' into 8311864
 - Initial commit

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/14831/files
  - new: https://git.openjdk.org/jdk/pull/14831/files/655442eb..e55dc5c1

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14831&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14831&range=02-03

  Stats: 4826 lines in 316 files changed: 2898 ins; 812 del; 1116 mod
  Patch: https://git.openjdk.org/jdk/pull/14831.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14831/head:pull/14831

PR: https://git.openjdk.org/jdk/pull/14831