std.unittests [updated] for review

[bearophile]

Andrei:

> Don, if you arrange things such that this user-level code:
> 
> int a = 42;
> double b = 3.14;
> assert(a <= b, "Something odd happened");
> 
> ultimately calls this runtime function:
> 
> assertCmpFailed("<=", "42", "3.14", "Something odd happened");
> 
> I promise I'll discuss with Sean and implement what it takes in druntime 
> to get that completed.

This makes me happier.

Bye,
bearophile