Feature/flat map

[carter-ya]

I also have another way of implementation:

// FlatMap returns a Stream that flattens the result of the given FlatMapFunc, which means it's a 1:N model.
func (s Stream) FlatMap(fn FlatMapFunc, opts ...Option) Stream {
	source := make(chan interface{})
	
	go func() {
		group := NewRoutineGroup()
		for item := range s.source {
			each := item
			group.Run(func() {
				otherStream := fn(each)
				for other := range otherStream.source {
					source <- other
				}
			})
		}
		group.Wait()
		close(source)
	}()
	
	return Range(source)
}

Ref: #4

[Copilot]

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