Design and Synthesis of 2′,3′‐Dideoxy‐2′,3′‐didehydro‐β‐L‐ cytidine (β‐L‐d4C) and 2′,3′‐Dideoxy‐2′,3′‐didehydro‐β‐L‐5‐ fluorocytidine (β‐L‐Fd4C), Two Exceptionally Potent Inhibitors of Human Hepatitis B Virus (HBV) and Potent Inhibitors of Human Immunodeficiency Virus (HIV) in vitro.