Rework of the typed Signal API:

- `Class::signals()` now takes self by reference (`&self`) instead of `&mut self`, similar changes to the `connect_**` methods. This was introduced to avoid `clone()` boilerplate when a class wants to connect to a signal that belongs to one of their fields.
- Replace trait `SignalReceiver` with direct usage of `FnMut($(Ps,)*)`. This fixes type inference issues with Rustc.
- Standardized `connect` methods in the Signal API (both `TypedSignal` and `ConnectBuilder`), changes:
    - `connect`: (unchanged).
    - `connect_self`: now accepts both engine and user classes (by using the new trait `GodotDeref`).
    - `connect_obj`: renamed to `connect_other`, now accepts both engine and user classes (by using the new trait `GodotDeref`).
    - `ConnectBuilder::function`: renamed to `connect`.
    - `ConnectBuilder::method_mut`: split into `connect_self` and `connect_other`.
    - `ConnectBuilder::method_immut`: removed.
    - `ConnectBuilder::done`: removed, building is finalized upon calling one of the `connect_**` methods.
- Removed type-state pattern on ConnectBuilder, and trimmed down generics.

Author: Houtamelo

godot-codegen/src/generator/signals.rs

@@ -120,9 +120,9 @@ fn make_with_signals_impl(
 
             // During construction, C = Self.
             #[doc(hidden)]
-            fn __signals_from_external(gd_mut: &mut Gd<Self>) -> Self::SignalCollection<'_, Self> {
+            fn __signals_from_external(gd_ref: & Gd<Self>) -> Self::SignalCollection<'_, Self> {
                 Self::SignalCollection {
-                    __internal_obj: Some(gd_mut.clone()),
+                    __internal_obj: Some(gd_ref.clone()),
                 }
             }
         }

godot-core/src/obj/gd.rs

@@ -732,7 +732,7 @@ where
     ///
     /// [`WithUserSignals::signals()`]: crate::obj::WithUserSignals::signals()
     #[cfg(since_api = "4.2")]
-    pub fn signals(&mut self) -> T::SignalCollection<'_, T> {
+    pub fn signals(&self) -> T::SignalCollection<'_, T> {
         T::__signals_from_external(self)
     }
 }

godot-core/src/obj/traits.rs

@@ -473,7 +473,7 @@ pub trait WithSignals: GodotClass + Inherits<crate::classes::Object> {
     ///
     /// Takes by reference and not value, to retain lifetime chain.
     #[doc(hidden)]
-    fn __signals_from_external(external: &mut Gd<Self>) -> Self::SignalCollection<'_, Self>;
+    fn __signals_from_external(external: &Gd<Self>) -> Self::SignalCollection<'_, Self>;
 }
 
 /// Implemented for user-defined classes with at least one `#[signal]` declaration.