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)