Make enum auto castable

[via Digitalmars-d-learn]

On Sun, Jun 04, 2017 at 08:15:46PM -0700, Jonathan M Davis via Digitalmars-d-learn wrote:
> Well, implicit conversions tend to cause a lot of bugs in C++ code, so
> Walter decided to restrict them quite a bit more in D than they are in C++.

So D has plenty of implicit conversion. It is implicit *construction* that it lacks.

And alas, C++'s mistake is implicit is default, so it gets used in places it wasn't intended. We should have just had explicit default with implicit as an option.