hashutil: new package for hashing-related features

Right now this just tries to use our sha1dc and if it's missing (eg a
--pure build) we fall back to hashlib. I imagine in the future we'll
want some other things in here for detecting what hasher is in use as
we transition off sha1.

Differential Revision: https://phab.mercurial-scm.org/D7848

mercurial/utils/hashutil.py

@@ -0,0 +1,9 b''
+from __future__ import absolute_import
+
+import hashlib
+
+try:
+    from mercurial.thirdparty import sha1dc
+    sha1 = sha1dc.sha1
+except (ImportError, AttributeError):
+    sha1 = hashlib.sha1