In code that supports both byte and unicode strings,
mixing unicode strings with plain character constants
is frequent. E.g. both sre_compile and xmlproc look for
specific characters in an input string. Every usage of
such a character requires default decoding, which will
create a temporary Unicode object.

This patch caches Unicode objects that represent ASCII
characters. On the benchmark

import time
u = u""
t=time.time()
for i in xrange(1000000):
    u+"("
print time.time()-t

it shows a 10% speed-up.