core: impl Value for &mut T where T: Value (tokio-rs#1385)

Impl `Value` for `&mut T` where `T: Value`. 

tokio-rs#1378 improve the `tracing::instrument` macro's recording behavior: all
primitive types implementing the `Value` trait will be recorded as
fields of that type instead of `fmt::Debug`. This PR is a prerequisite
for those `&mut` function arguments to achieve that.

tracing-core/src/field.rs

@@ -433,6 +433,19 @@ where
     }
 }
 
+impl<'a, T: ?Sized> crate::sealed::Sealed for &'a mut T where T: Value + crate::sealed::Sealed + 'a {}
+
+impl<'a, T: ?Sized> Value for &'a mut T
+where
+    T: Value + 'a,
+{
+    fn record(&self, key: &Field, visitor: &mut dyn Visit) {
+        // Don't use `(*self).record(key, visitor)`, otherwise would
+        // cause stack overflow due to `unconditional_recursion`.
+        T::record(self, key, visitor)
+    }
+}
+
 impl<'a> crate::sealed::Sealed for fmt::Arguments<'a> {}
 
 impl<'a> Value for fmt::Arguments<'a> {