rust-cpython: consistency in use of hg-core constructs

rust-cpython: consistency in use of hg-core constructs

While not everybody likes the `CoreLazy` and `CoreIterator`
aliases, it's better not to mix them with direct references.
Note: it's quite possible in the future that these would stop
being exposed at the top of the `hg` crate

Differential Revision: https://phab.mercurial-scm.org/D5547

--- a/rust/hg-cpython/src/ancestors.rs	Sat Dec 22 11:38:03 2018 +0100
+++ b/rust/hg-cpython/src/ancestors.rs	Tue Jan 08 13:54:01 2019 +0100
@@ -27,7 +27,6 @@
     ObjectProtocol, PyClone, PyDict, PyModule, PyObject, PyResult, Python,
 };
 use exceptions::GraphError;
-use hg;
 use hg::Revision;
 use hg::{AncestorsIterator as CoreIterator, LazyAncestors as CoreLazy};
 use std::cell::RefCell;
@@ -66,9 +65,9 @@
     def __new__(_cls, index: PyObject, initrevs: PyObject, stoprev: Revision,
                 inclusive: bool) -> PyResult<AncestorsIterator> {
         let initvec = reviter_to_revvec(py, initrevs)?;
-        let ait = match hg::AncestorsIterator::new(Index::new(py, index)?,
-                                                   initvec, stoprev,
-                                                   inclusive) {
+        let ait = match CoreIterator::new(Index::new(py, index)?,
+                                          initvec, stoprev,
+                                          inclusive) {
             Ok(ait) => ait,
             Err(e) => {
                 return Err(GraphError::pynew(py, e));