Extend the compact gate implementation to provide index method

This method essentially returns the unique index associated with the current gate. The plan is to use that number for metrics uniqueness check: they greatly benefit from knowing that the dimension for compact gate is `u64`

ipa-step-derive/src/lib.rs

@@ -165,6 +165,18 @@ fn derive_gate_impl(ast: &DeriveInput) -> TokenStream {
                 <Self as ::std::fmt::Display>::fmt(self, f)
             }
         }
+
+        impl #name {
+            /// Returns the current index. It matches the index of the latest step
+            /// this gate has been narrowed to.
+            ///
+            /// If gate hasn't been narrowed yet, it returns the index of the default value.
+            #[must_use]
+            pub fn index(&self) -> ::ipa_step::CompactGateIndex {
+                self.0
+            }
+        }
+
     };
 
     // This environment variable is set by build scripts,

ipa-step-test/src/lib.rs

@@ -17,22 +17,35 @@ mod tests {
 
     #[test]
     fn narrows() {
+        assert_eq!(ComplexGate::default().index(), 0);
         assert_eq!(ComplexGate::default().as_ref(), "/");
         assert_eq!(
             ComplexGate::default().narrow(&ComplexStep::One).as_ref(),
             "/one"
         );
+        assert_eq!(ComplexGate::default().narrow(&ComplexStep::One).index(), 1,);
         assert_eq!(
             ComplexGate::default().narrow(&ComplexStep::Two(2)).as_ref(),
             "/two2"
         );
+        assert_eq!(
+            ComplexGate::default().narrow(&ComplexStep::Two(2)).index(),
+            10,
+        );
         assert_eq!(
             ComplexGate::default()
                 .narrow(&ComplexStep::Two(2))
                 .narrow(&BasicStep::One)
                 .as_ref(),
             "/two2/one"
         );
+        assert_eq!(
+            ComplexGate::default()
+                .narrow(&ComplexStep::Two(2))
+                .narrow(&BasicStep::One)
+                .index(),
+            11,
+        );
         assert_eq!(
             ComplexGate::from("/two2/one"),
             ComplexGate::default()