Refactor tail call resolution logic

The tail call resolution logic was duplicated in `Environment::call` and
`VM::run`. This commit extracts the tail call resolution logic into a
single method `VM::resolve_tail_calls` and uses it in both places.

Additionally, this commit adds support for series indexing as a form of
tail call, allowing for direct access to series elements through the
`series(index)` syntax. This is useful for back-referencing in
time-series data.

A new example `err.myc` is added to demonstrate basic series usage and
error handling.

src/ast/environment.rs

@@ -501,7 +501,10 @@ impl Environment {
                         Ok(v) => v,
                         Err(e) => panic!("Myc Runtime Error: {}", e),
                     };
-                    vm.resolve_tail_calls(res)
+                    match vm.resolve_tail_calls(&mut crate::ast::vm::NoOpObserver, Ok(res)) {
+                        Ok(v) => v,
+                        Err(e) => panic!("Myc Runtime Error: {}", e),
+                    }
                 }),
             });
         }
@@ -528,7 +531,10 @@ impl Environment {
                         Err(e) => panic!("Myc Runtime Error (Closure): {}", e),
                     };
                 }
-                vm.resolve_tail_calls(final_res)
+                match vm.resolve_tail_calls(&mut crate::ast::vm::NoOpObserver, Ok(final_res)) {
+                    Ok(v) => v,
+                    Err(e) => panic!("Myc Runtime Error: {}", e),
+                }
             }),
         })
     }
@@ -645,23 +651,6 @@ impl Environment {
             result = vm.run_with_observer(&mut observer, &closure.exec_node);
         }
 
-        while let Ok(Value::TailCallRequest(payload)) = result {
-            let (next_obj, next_args) = *payload;
-            if next_obj
-                .as_any()
-                .downcast_ref::<crate::ast::vm::Closure>()
-                .is_some()
-            {
-                result = vm.run_with_args_observed(&mut observer, next_obj, &next_args);
-            } else {
-                result = Err(format!(
-                    "Tail call target is not a closure: {}",
-                    next_obj.type_name()
-                ));
-                break;
-            }
-        }
-
         self.run_pipeline();
 
         Ok((result, observer.logs))