rust-index: moved constructor in separate impl block

with the upcoming addition of the nodemap logic, it's going to become
more complicated.

Being in the separate impl block has the advantage to be subject to rustfmt,
whereas code inside macro calls isn't.

Differential Revision: https://phab.mercurial-scm.org/D8153

rust/hg-cpython/src/revlog.rs

@@ -28,8 +28,7 b' py_class!(pub class MixedIndex |py| {'
     data cindex: RefCell<cindex::Index>;
 
     def __new__(_cls, cindex: PyObject) -> PyResult<MixedIndex> {
-        Self::create_instance(py, RefCell::new(
-            cindex::Index::new(py, cindex)?))
+        Self::new(py, cindex)
     }
 
     /// Compatibility layer used for Python consumers needing access to the C index
@@ -199,6 +198,13 b' py_class!(pub class MixedIndex |py| {'
 });
 
 impl MixedIndex {
+    fn new(py: Python, cindex: PyObject) -> PyResult<MixedIndex> {
+        Self::create_instance(
+            py,
+            RefCell::new(cindex::Index::new(py, cindex)?),
+        )
+    }
+
     /// forward a method call to the underlying C index
     fn call_cindex(
         &self,