"whether Congress, in enacting § 1979, meant to give a remedy to parties deprived
of constitutional rights, privileges and immunities by an official's abuse of
his position. Cf. Williams v. United
States
, 341 U.S. 97; Screws v. United States
, 325 U.S. 91; United States v. Classic
, 313 U.S. 299. We
conclude that it did so intend."