Glycosidase‐Catalyzed Oligosaccharide Synthesis: Preparation of the N‐ Acetylchitooligosaccharides Penta‐N‐acetylchitopentaose and Hexa‐N‐ acetylchitohexaose Using the β‐N‐Acetylhexosaminidase of Aspergillus oryzae.