Fix a buglet in CorePrep:  an expression such as

	case __coerce (\x -> e) :: T of { ... }

would be left as is, but the lambda expression should really be
abstracted as a let (causes a panic later in srtExpr; shown up by
the dynamic001 test).  There was a missing call to deLam in the case
for Case expressions in corePrepExprFloat.

In addition, I made a new version of deLam, deLamFloat, which can
float any bindings generated.  This helps to generate slightly cleaner
code in the above case (the binding is floated out of the scrutinee).

Also: GC unused imports while I'm here.