SpecConstr now specialises on constants and lambdas

Roman inspired me to beef up SpecConstr to deal with
a) constant arguments
b) lambda arguments

This is described in elaborate comments in the file:
 	Note [Specialising for constant parameters]
	Note [Specialising for lambda parameters]

I also took the opportunity to fix the usage analysis done by
SpecConstr, which was only handling the top-level correctly.
Now it does nesting too.