[Bug #536241] string.zfill() produces mangled output for a Unicode string.

Walter Doerwald provided a patch, which I've modified in two ways:

1) (Uncontroversial) Removed code to make module work in earlier versions of
   Python without the unicode() built-in

2) (Poss. controversial) Instead of making string.zfill take the repr()
   of non-string objects, take the str().

Should a warning be added to this branch of the code so that the automatic
str() can be deprecated?

2.2.2 bugfix candidate, assuming the repr()->str() change is deemed OK.