+March 97: setStgVarInfo guarantees to leave every variable's arity correctly
+set. The lambda lifter makes some let-bound variables (which have arities)
+and turns them into lambda-bound ones (which should not, else we get Vap trouble),
+so this guarantee is necessary, as well as desirable.
+
+The arity information is used in the code generator, when deciding if
+a right-hand side is a saturated application so we can generate a VAP
+closure.
+