* string_contains now calls PyUnicode_Contains() only when the other
operand is a Unicode string (not whenever it's not a string).
* New format style '%r' inserts repr(arg) instead of str(arg).
* '...%s...' % u"abc" now coerces to Unicode just like
string methods. Care is taken not to reevaluate already formatted
arguments -- only the first Unicode object appearing in the
argument mapping is looked up twice. Added test cases for
this to test_unicode.py.