We've been allocating slice objects for slicing operations, but
CPython's internal slice methods already take separate start+stop
arguments.  So if we see a slice, instead of creating the slice
and sending it off to getitem, try calling PySequence_GetSlice.

This will be slower for classes with user-defined __getitem__
functions that can handle slices; we can fix that by adding rewriting
to this new endpoint, but it seems to not matter too much right now.