Implement withFirst(): extract first stream element

[amaembo]

Sometimes it's useful to extract the first stream element and use it to process other elements. Some solutions are proposed and discussed here. In StreamEx we must make such operation lazy: it should not consume any stream elements until the terminal operation is performed.

Two possible versions are proposed:

class StreamEx<T> {
    // Return EntryStream where keys are the same and represent the first stream element
    // while values are the rest elements (the stream length is one element less than 
    // the original stream)
    public EntryStream<T, T> withFirst() {...};

    // Return StreamEx which elements are the result of applying the mapper function 
    // to the first stream element and every other element
    <U> StreamEx<U> withFirst(BiFunction<T, T, U> mapper) { ... };
}

The biggest problem is the current flatMap bug: this code must produce 1, 2, 3, 4, 5

StreamEx.of(0, 2, 4).flatMap(x -> Stream.of(x, x + 1)).parallel()
              .withFirst().values().toList();

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Issue #54 extracted from this one as it's substantially different.