Avoid raw pointers in SliceBox and make the Send bound explicit

[adamreichold]

This is a follow-up to #189. I think the main benefit besides removing two unsafe usages is propagating the Send bound which seems to imply that Element must indeed be Send to be wrapped within SliceBox and managed by Python.

Taking this a bit further, there seems to be nothing slice or box specific about SliceBox, i.e. we could just as well have a

struct Opaque<T>(pub(crate) T);

which does not care about its contents at all, that is besides making it possible for Python to indirectly call T's Drop implementation.

[davidhewitt]

making it possible for Python to indirectly call T's Drop implementation

This could also be done by using Python's capsule C API, which we could expose in PyO3. PyO3/pyo3#1193

[kngwyu]

I agree with the change, since anyway all types we currently support as dtype require Send!

Taking this a bit further, there seems to be nothing slice or box specific about SliceBox, i.e. we could just as well have a

Yeah, but somehow NumPy's inheritance mechanism is very tricky and I'm not sure this is resuable.