feat: extract serde_core out of serde

serde_core/src/integer128.rs

@@ -0,0 +1,9 @@
+// No longer used. Old versions of serde used this macro for supporting targets
+// that did not yet have 128-bit integer support.
+#[macro_export]
+#[doc(hidden)]
+macro_rules! serde_if_integer128 {
+    ($($tt:tt)*) => {
+        $($tt)*
+    };
+}