Updated patch with optimization for:
 * rfind
 * rindex
 * rsplit
 * rpartition

And an optimization was implemented in r46398 but never used because
"#define USE_FAST" was removed 2 hours before: r46366.
==> I changed this to activate optimization on "S.split" too.

I propose to commit these changes before looking for further improvements.
(I plan to refactor some functions and macro to Objects/stringlib)