Truly algebraic Variant and Nullable

[9il]

Truly algebraic Variant and Nullable with an order-independent 
list of types.

Nullable is defined as
```
alias Nullable(T...) = Variant!(typeof(null), T);
```

Variant and Nullable with zero types are allowed.

`void` type is supported.

Visitors are allowed to return different types.

Cyclic referencing between different variant types are supported.

More features and API:

http://mir-core.libmir.org/mir_algebraic.html

Cheers,
Ilya

The work has been sponsored by Kaleidic Associates and Symmetry 
Investments.