Provide warning for defs that are used only in (mutual) recursion

This patch provides errors for defs that are used only in
possibly-mutual recursion, and are not reachable outside of their
recursive closures. For example:

```
test_report!(
    mutual_recursion_not_reached_nested,
    indoc!(
        r#"
        app "test" provides [main] to "./platform"

        main =
            f = \{} -> if Bool.true then "" else g {}
            g = \{} -> if Bool.true then "" else f {}
            ""
        "#
    ),
@r###"
── DEFINITIONs ONLY USED IN RECURSION ──────────────────── /code/proj/Main.roc ─

These 2 definitions are only used in mutual recursion with themselves:

4│>      f = \{} -> if Bool.true then "" else g {}
5│>      g = \{} -> if Bool.true then "" else f {}

If you don't intend to use or export any of them, they should all be
removed!
"###
);
```

crates/compiler/mono/src/borrow.rs

@@ -68,7 +68,7 @@ pub fn infer_borrow<'a>(
 
     let sccs = matrix.strongly_connected_components_all();
 
-    for group in sccs.groups() {
+    for (group, _) in sccs.groups() {
         // This is a fixed-point analysis
         //
         // all functions initiall own all their parameters