  The paper has been withdrawn by the author, due a gap in the proof of Theorem
6.1. The gap was discovered by M. Van den Bergh.
  Theorem 6.1 is used to prove the main result of the paper, namely Theorem 0.7
(decomposition in arbitrary characteristic). At this time we do not have an
alternate proof.
  Other results of the paper (Theorems 0.2, 0.3, 0.6 and 3.1) are not effected
by this problem, and they remain valid.
