"Recursive functions" are those functions f that might call either:
themselves, or
another function that in turn might call f.
For example in the following simultaneous definition of fg and h, f and g are recursive functions, but not h: let rec f x = g x
and g x = f x
and h x = g x
Determine the recursive functions, if any, bound by the given set of function declarations. This is only intended to be used by Flambda.create_function_declarations.