Add HPy_(Get|Set|Del)Slice

[fangerer]

Again, we needed this for Cython/HPy (in particular, for the fannkuch benchmark) because getting and setting slices required multiple upcalls like this:

// create a slice object
HPy start = HPyLong_FromLong(lstart);
HPy stop = HPyLong_FromLong(lsop);
HPy ctor_args[] = { start, stop };
HPy slice = HPy_Call(ctx, ctx->h_SliceType, ctor_args, 2, HPy_NULL);
HPy res = HPy_GetItem(ctx, seq, slice);
HPy_Close(ctx, slice);
HPy_Close(ctx, stop);
HPy_Close(ctx, start);

Now, this can be done with:

HPy res = HPy_GetSlice(ctx, seq, lstart, lstop);

The proposed APIs are probably worth some discussion. E.g. we may think about introducing a third argument to each denoting the slice's step. Right now, this will always be 1.