Add PartialEq and Eq implementations for Span
diff --git a/src/stable.rs b/src/stable.rs
index 5421060..cb376a9 100644
--- a/src/stable.rs
+++ b/src/stable.rs
@@ -311,7 +311,7 @@
     }
 }
 
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub struct Span {
     #[cfg(procmacro2_semver_exempt)]
     lo: u32,