rephrase type irgen hook -> irext, to eliminate cases where it'd be very easy to accidentally cause infinite recursion of ir generation (e.g. slice(x) requiring evaluation of 'lhs.count', where lhs == slice(x)). also bring back slice hooks

src/eval_visualization/eval_visualization_core.h

@@ -256,7 +256,7 @@ struct EV_StringIterTask
   E_Eval eval;
   U64 idx;
   S32 depth;
-  B32 redirect_array_to_sets_and_structs;
+  B32 redirect_to_sets_and_structs;
   void *user_data;
 };