RFR: 8267452: Delegate forEachRemaining in Spliterators.iterator() [v2]

[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).

Tagir F. Valeev has updated the pull request incrementally with one additional commit since the last revision:

  Test: formatting; tests for empty spliterator

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4124/files
  - new: https://git.openjdk.java.net/jdk/pull/4124/files/5b05135f..9a5e0dd2

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4124&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4124&range=00-01

  Stats: 109 lines in 1 file changed: 30 ins; 14 del; 65 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