Change MsgMut's _mut() accessors to use (&mut self) instead of (&self)

If this took a &self there were holes in thread and memory safety, because it's allowed to get multiple &MsgMuts (but only &mut MsgMuts).

PiperOrigin-RevId: 596991263

rust/test/shared/simple_nested_test.rs

@@ -73,7 +73,7 @@ fn test_nested_muts() {
 }
 
     let mut outer_msg = Outer::new();
-    let inner_msg: InnerMut<'_> = outer_msg.inner_mut();
+    let mut inner_msg: InnerMut<'_> = outer_msg.inner_mut();
     assert_that!(
         inner_msg,
         matches_pattern!(InnerMut{

src/google/protobuf/compiler/rust/message.cc

@@ -304,7 +304,7 @@ void GetterForViewOrMut(Context& ctx, const FieldDescriptor& field,
                // TODO: check mutational pathway genn'd correctly
                if (is_mut) {
                  ctx.Emit({}, R"rs(
-                  pub fn r#$field$_mut(&self) -> $pb$::Mut<'_, $RsType$> {
+                  pub fn r#$field$_mut(&mut self) -> $pb$::Mut<'_, $RsType$> {
                     static VTABLE: $pbi$::$vtable$$optional_type_args$ =
                       $pbi$::$vtable$::new(
                         $pbi$::Private,