Should not_null act like an alias

[trebconnell]

Right now it's implemented like a smart pointer, with method like get(), but if I'm reading the isocpp guidelines correctly it should act like an alias. Given that it's under GSL.Views with owner and not GSL.Owner with the smart pointers.

template <class T> using not_null = T;

[trebconnell]

The thinking behind the guideline being that you may want to use these types on an ABI interface. Having them be a class at all would prohibit this. Is the plan to replace the dbg checking of these types with the static analysis and change them into aliases in the long-term?

[gdr-at-ms]

We actually want not_null to be a distinct type. It has an invariant deterministically expressible and cheap to enforce.