Fix the string methods that implement slice-like semantics with
optional args (count, find, endswith, etc.) to properly handle
indeces outside [INT_MIN, INT_MAX]. Previously the "i" formatter
for PyArg_ParseTuple was used to get the indices. These could overflow.

This patch changes the string methods to use the "O&" formatter with
the slice_index() function from ceval.c which is used to do the same
job for Python code slices (e.g. 'abcabcabc'[0:1000000000L]). slice_index()
is renamed _PyEval_SliceIndex() and is now exported. As well, the return
values for success/fail were changed to make slice_index directly
usable as required by the "O&" formatter.

[GvR: shouldn't a similar patch be applied to unicodeobject.c?]