RFR: 8327858: Improve spliterator and forEach for single-element immutable collections

[Chen Liang]

Please review this patch that:
1. Implemented `forEach` to optimize for 1 or 2 element collections.
2. Implemented `spliterator` to optimize for a single element.

The default implementations for multiple-element immutable collections are fine as-is, specializing implementation doesn't provide much benefit.

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Commit messages:
 - Null checks should probably be in the beginning...
 - mark implicit null checks
 - Merge branch 'master' of https://github.com/openjdk/jdk into feature/imm-coll-stream
 - Copyright year, revert changes for non-few element collections
 - Merge branch 'master' of https://github.com/openjdk/jdk into feature/imm-coll-stream
 - Merge branch 'feature/imm-coll-stream' of https://github.com/liachmodded/jdk into feature/imm-coll-stream
 - Spliterator for 12, iterate/forEach benchmark
 - fix comments
 - Remove useless implementations
 - Implement forEach for immutable collections
 - ... and 1 more: https://git.openjdk.org/jdk/compare/d451f818...8036e9e6

Changes: https://git.openjdk.org/jdk/pull/15834/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15834&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8327858
  Stats: 91 lines in 2 files changed: 89 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/15834.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15834/head:pull/15834

PR: https://git.openjdk.org/jdk/pull/15834