2′-fluoromaltose: Synthesis and properties of 4-O-(2-deoxy-2-fluoro-α-d-glucopyranosyl)-d-glucopyranose, and the crystal structure of 2,3-di-O-acetyl-1,6-anhydro-4-O-(3,4-tri-O-acetyl-2-deoxy-2-fluoro-α-d-glucopyranosyl)-β-d- glucopyranose