Small fixes after #[cfg(since_api = "4.2")] is part of clippy/test

Some problems were previously undetected, since CI ran with 4.1.1-stable, excluding newer code paths.

Concretely:
- Fix Latin-1 character incorrectly represented in string lateral.
- Clippy lints related to Callable and its tests.

Author: Bromeon

godot-core/src/builtin/callable.rs

@@ -375,6 +375,7 @@ mod custom_callable {
         ///
         /// Return `Ok(...)` if the call succeeded, and `Err(())` otherwise.
         /// Error handling is mostly needed in case argument number or types mismatch.
+        #[allow(clippy::result_unit_err)] // TODO remove once there's a clear error type here.
         fn invoke(&mut self, args: &[&Variant]) -> Result<Variant, ()>;
     }
 

godot-core/src/builtin/string/string_name.rs

@@ -36,7 +36,8 @@ impl StringName {
     /// ```no_run
     /// use godot::builtin::StringName;
     ///
-    /// let sname = StringName::from_latin1_with_nul(b"± Latin-1 string\0");
+    /// // '±' is a Latin-1 character with codepoint 0xB1. Note that this is not UTF-8, where it would need two bytes.
+    /// let sname = StringName::from_latin1_with_nul(b"\xb1 Latin-1 string\0");
     /// ```
     ///
     /// # Panics

itest/rust/src/builtin_tests/containers/callable_test.rs

@@ -220,7 +220,11 @@ mod custom_callable {
         let b = Callable::from_custom(Adder::new_tracked(3, bt.clone()));
         let c = Callable::from_custom(Adder::new_tracked(4, ct.clone()));
 
-        assert_eq!(a, a);
+        // clippy complains with assert_eq!(a, a) and it can't be suppressed with the suggested #[allow(clippy::eq_op)], hence this ceremony.
+        #[allow(clippy::eq_op)]
+        if a != a {
+            panic!("a != a; left: {a:?}, right: {a:?}");
+        }
         assert_eq!(
             eq_count(&at),
             0,