Teach SpecConstr how to handle mutually-recursive functions

Teach SpecConstr how to handle mutually-recursive functions

Roman found cases where it was important to do SpecConstr for
mutually-recursive definitions.  Here is one:
	foo :: Maybe Int -> Int
	foo Nothing  = 0
	foo (Just 0) = foo Nothing
	foo (Just n) = foo (Just (n-1))
By the time SpecConstr gets to it, it looks like this:
	lvl = foo Nothing
	foo Nothing  = 0
	foo (Just 0) = lvl
	foo (Just n) = foo (Just (n-1))

Happily, it turns out to be rather straightforward to generalise the
transformation to mutually-recursive functions.  Look, ma, only 4
extra lines of ocde!