Add enums and unions

[matt-cornell]

These are tagged enums and unions. They're basically the same, the only difference is that unions have a sorted order, so @union | a | b and @union | b | a are the same type, while @enum | a | b and @enum | b | a are not.
These take place at the type level, and are made to be used with symbols. For example, a unit enum can be created like this:

const my_unit_enum = @enum
| $value1
| $value2
| $value3;

With this, enums with fields can also be created:

const my_field_enum = @enum
| ($int, i64)
| ($float, f64);