RFR: 8267452: Delegate forEachRemaining in Spliterators.iterator()

[Tagir F.Valeev]

Sometimes, `Spliterator::forEachRemaining` has much more optimized implementation, compared to `Spliterator::tryAdvance`. For example, if a `Stream::spliterator` called for a stream that has intermediate operations, `tryAdvance()` may buffer intermediate elements while `forEachRemaining()` just delegates to the `wrapAndCopyInto` (see `StreamSpliterators.AbstractWrappingSpliterator` and its inheritors).

`Spliterators::iterator` methods (used in particular by `Stream::iterator`) provide adapter iterators that delegate to the supplied spliterator. Unfortunately, they don't have a specialized implementation for `Iterator::forEachRemaining`. As a result, a default implementation is used that delegates to `hasNext`/`next`, which in turn causes the delegation to `tryAdvance`. It's quite simple to implement `Iterator::forEachRemaining` here, taking advantage of possible spliterator optimizations if the iterator client decides to use `forEachRemaining`.

This patch implements Iterator::forEachRemaining in Spliterators::iterator methods. Also, I nullize the `nextElement` in `Iterator::next` to avoid accidental prolongation of the user's object lifetime when iteration is finished. Finally, a small cleanup: I added the `final` modifier where possible to private fields in `Spliterators`.

Test-wise, some scenarios are already covered by SpliteratorTraversingAndSplittingTest. However, the resulting iterator is always wrapped into `Spliterators::spliterator`, so usage scenarios are somewhat limited. In particular, calling `hasNext` (without `next`) before `forEachRemaining` was not covered there. I added more tests in `IteratorFromSpliteratorTest` to cover these scenarios. I checked that removing `valueReady = false;` or `action.accept(t);` lines from newly implemented `forEachRemaining` method causes new tests to fail (but old tests don't fail due to this).

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Commit messages:
 - 8267452: Linebreaks fixed
 - 8267452: @bug tag
 - 8267452: Delegate forEachRemaining in Spliterators.iterator()

Changes: https://git.openjdk.java.net/jdk/pull/4124/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4124&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8267452
  Stats: 178 lines in 2 files changed: 173 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4124.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4124/head:pull/4124

PR: https://git.openjdk.java.net/jdk/pull/4124