I'm not sure about the name ObjSlice; the Obj sound redundant... The builtin `slice` is lowercase; technically we're not shadowing any builtins if we use `Slice` (though I can see it being confusing regardless.

I also think that this should be in `nengo/objects.py`, as `helpers.py` is kind of "unsupported", in the sense that nothing in helpers should be absolutely essential for Nengo to function, they just make things a bit easier sometimes. Slices are more important than that! They're super important.

If they're in `objects.py`, that gives us another possibility, which is `_Slice`, as they're not objects that we would want users using directly; they wouldn't be imported in `__init__.py` or anything like that.