py3: make py3 compat.iterbytestr simpler and faster

With Python 3.4.3, timit says 11.9 usec-> 6.44 usec. With Python
3.6.0, timeit says 14.1 usec -> 9.55 usec.

mercurial/pycompat.py

@@ -78,7 +78,7 b' if ispy3:'
 
     def iterbytestr(s):
         """Iterate bytes as if it were a str object of Python 2"""
-        return iter(s[i:i + 1] for i in range(len(s)))
+        return map(bytechr, s)
 
     def sysstr(s):
         """Return a keyword str to be passed to Python functions such as