`Find_recursive_functions`

"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 `f`

`g`

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
```

```
val in_function_declarations :
Flambda.function_declarations ->
backend:(module Backend_intf.S) ->
Variable.Set.t
```

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`

.