Nucleic‐Acid Analogs with Restricted Conformational Flexibility in the Sugar‐Phosphate Backbone (‘Bicyclo‐DNA’). Part 5. Synthesis, Characterization, and Pairing Properties of Oligo‐α‐D‐(bicyclodeoxynucleotides) of the Bases Adenine and Thymine (α‐Bicyclo‐DNA)