Split autodiff into autodiff_forward and autodiff_reverse

Pending fix.
```
error: cannot find a built-in macro with name `autodiff_forward`
    --> library\core\src\macros\mod.rs:1542:5
     |
1542 | /     pub macro autodiff_forward($item:item) {
1543 | |         /* compiler built-in */
1544 | |     }
     | |_____^

error: cannot find a built-in macro with name `autodiff_reverse`
    --> library\core\src\macros\mod.rs:1549:5
     |
1549 | /     pub macro autodiff_reverse($item:item) {
1550 | |         /* compiler built-in */
1551 | |     }
     | |_____^

error: could not compile `core` (lib) due to 2 previous errors
```

Author: Sa4dUs

core/src/lib.rs

@@ -229,7 +229,7 @@ pub mod assert_matches {
 /// Unstable module containing the unstable `autodiff` macro.
 pub mod autodiff {
     #[unstable(feature = "autodiff", issue = "124509")]
-    pub use crate::macros::builtin::autodiff;
+    pub use crate::macros::builtin::{autodiff_forward, autodiff_reverse};
 }
 
 #[unstable(feature = "contracts", issue = "128044")]

core/src/macros/mod.rs

@@ -1536,6 +1536,20 @@ pub(crate) mod builtin {
         /* compiler built-in */
     }
 
+    #[unstable(feature = "autodiff", issue = "124509")]
+    #[allow_internal_unstable(rustc_attrs)]
+    #[rustc_builtin_macro]
+    pub macro autodiff_forward($item:item) {
+        /* compiler built-in */
+    }
+
+    #[unstable(feature = "autodiff", issue = "124509")]
+    #[allow_internal_unstable(rustc_attrs)]
+    #[rustc_builtin_macro]
+    pub macro autodiff_reverse($item:item) {
+        /* compiler built-in */
+    }
+
     /// Asserts that a boolean expression is `true` at runtime.
     ///
     /// This will invoke the [`panic!`] macro if the provided expression cannot be