Add MoreCollectors.toBatches collector

[amaembo]

There's repeating requests to add a partitioning of the stream to the fixed size batches. See, for example, this question. Some problems can be solved using StreamEx.ofSubLists, but sometimes the source is not a random access list. It's hardly possible to create efficient parallel implementation for ordered batches, but if we don't guarantee an order (any subset of input elements may be collected into single batch), then implementation becomes quite straightforward. Probably it's a good idea to add such collector.

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