Direct Synthesis of β-Mannans. A Hexameric [→3)-β-d-Man-(1→4)-β-d-Man-(1]3 Subunit of the Antigenic Polysaccharides from Leptospira biflexa and the Octameric (1→2)-Linked β-d-Mannan of the Candida albicans Phospholipomannan. X-ray Crystal Structure of a Protected Tetramer