Support fixpoint-fixing under recursion variables

Sometimes, we might need to fixpoint-fix a unification like

[ Bar [ Bar <a>, Foo ], Foo ] as <a>  🛠️  [ Bar <b>, Foo ] as <b>

where we hit a comparison between <a> and <b>. In this case, follow each
recursion point independently and see if we can find the chain to the needle
we were searching for.

Closes #5476

crates/compiler/debug_flags/src/lib.rs

@@ -92,6 +92,9 @@ flags! {
     /// Prints all type mismatches hit during type unification.
     ROC_PRINT_MISMATCHES
 
+    /// Prints all type variables entered for fixpoint-fixing.
+    ROC_PRINT_FIXPOINT_FIXING
+
     /// Verifies that after let-generalization of a def, any rigid variables in the type annotation
     /// of the def are indeed generalized.
     ///