Optimize pipeline creation

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Each new use of |then>> creates a separate instance of ActionChain, which must each time combine all functions in a pipeline into a separate function and since it does this using bind, then only the number of calls to bind is n**2 / 2, where n is the number of atomic functions in a pipeline.

Head-on solution: initialize _main_action only if a pipeline (ActionChain) is called and add a special initialization mode for ActionChain when it is created via |then>> operator (_ActionChainInfix).