encoding.lower: use fast ASCII lower

This benefits, among other things, the case collision auditor.

On a Linux system with a large real-world repo where all filenames are ASCII,
hg perfcca:

before: wall 0.260157 comb 0.270000 user 0.230000 sys 0.040000 (best of 38)
after:  wall 0.164616 comb 0.160000 user 0.160000 sys 0.000000 (best of 54)

mercurial/encoding.py

@@ -270,8 +270,7 b" asciilower = getattr(parsers, 'asciilowe"
 def lower(s):
     "best-effort encoding-aware case-folding of local string s"
     try:
-        s.decode('ascii') # throw exception for non-ASCII character
-        return s.lower()
+        return asciilower(s)
     except UnicodeDecodeError:
         pass
     try: