fix ticket 636: build the correct slice objects on 2-value object slicing
fix negative slice indices
implement correct slicing for setslice and delslice

--HG--
rename : tests/compile/delslice.pyx => tests/run/delslice.py
changelog
fix passing explicit None slice indices into the getslice code, enable test case for ticket 636

--HG--
rename : tests/run/slice2_T636.pyx => tests/run/slice2_T636.py
add tags to tests
avoid code duplication by generating GetSlice/SetSlice utility code from a common template
avoid code duplication by generating List/Tuple_GetSlice utility code from a common template
honour wraparound option in object slicing code
minor code cleanup