Refactor OccAnal; and improve dead-code elimination

The occurrence analyer is now really rather subtle when dealing
with recursive groups; see Note [Loop breaking and RULES] especially.

This patch refactors this code a bit, notably
  * Introduces a new data type Details instead of a tuple

  * More clearly breaks up a recursive group into its SCCs
	before processing it in a separate function occAnalRec

  * As a result, does better dead-code elimination, becuause it's
   	done per SCC rather than for the whole Rec