Improves IonContainer hashCode performance by ~30%.

[tgregg]

Description of changes:

• IonValueLite.java — refactored containerHashCode() to use parallel arrays (valueHashes, parents, iterators) instead of per-frame HashHolder objects, and converted HashHolder's instance methods into static helpers (hashStructField, updateHash). The hashes produced are the same.
• New IonContainerHashCodeBenchmark.java — JMH benchmark exercising monomorphic vs. megamorphic container hashing.

Results:

Benchmark                        poison    Before (ns/op)    After (ns/op)    Improvement
  containerHashCode_megamorphic    true      1943 ± 212        1542 ± 30        -21%
  containerHashCode_megamorphic    false     1944 ± 240        1510 ± 6         -22%
  containerHashCode_monomorphic    true      1738 ± 10         949 ± 9          -45%
  containerHashCode_monomorphic    false     1401 ± 57         939 ± 15         -33%

Container: IonList with 120 elements (monomorphic = all IonString; megamorphic = 6 types × 20)
JVM: Corretto 21, aarch64, -Xms3g -Xmx3g -XX:+UseParallelGC
poison=true pre-invokes hashCode() 40,000 times on the megamorphic container to simulate
real-world call-site pollution (JDK-8368292).

Credit: John Engebretson

Related: https://bugs.openjdk.org/browse/JDK-8368292

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.