Implement equality for strings

hornbeam_interpreter/src/engine.rs

@@ -597,7 +597,9 @@ impl<'a, O: OutputSystem + Send, LS: LocalisationSystem + Sync + Send> Interpret
                                 location: loc.clone(),
                             })
                         }
-                    },
+                    }
+                    (Value::Str(lstr), Value::Str(rstr)) => Ok(Value::Bool(lstr == rstr)),
+                    // TODO List vs List. Not *that* useful but can be occasionally. But it does involve recursion.
                     (lother, rother) => Err(InterpreterError::TypeError {
                         context: "Equals".to_string(),
                         conflict: format!("can't test {lother:?} and {rother:?} for equality!"),