Avoid creating most slice objects

[kmod]

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.

           django_template3.py             2.4s (4)             2.4s (4)  +0.5%
                 pyxl_bench.py             2.1s (2)             2.1s (2)  +0.8%
     sqlalchemy_imperative2.py             2.5s (2)             2.5s (2)  -0.3%
       django_template3_10x.py            14.8s (2)            14.8s (2)  +0.2%
             pyxl_bench_10x.py            15.7s (2)            15.1s (2)  -3.6%
 sqlalchemy_imperative2_10x.py            18.7s (2)            18.5s (2)  -0.9%
                       geomean                 6.2s                 6.1s  -0.6%

[undingen]

lgtm

[kmod]

thanks for taking a look :)