RFR: 8319123: Implement JEP 461: Stream Gatherers (Preview) [v11]

[Alan Bateman]

On Fri, 17 Nov 2023 11:28:23 GMT, Viktor Klang <vklang at openjdk.org> wrote:

>> This Pull-Request implements [JEP-461](https://openjdk.org/jeps/461)
>
> Viktor Klang has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Adding @implSpec for Gatherer.andThen, plus adds tests to verify the default impls for Gatherer-related interfaces

Marked as reviewed by alanb (Reviewer).

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

PR Review: https://git.openjdk.org/jdk/pull/16420#pullrequestreview-1736737940