Two things: recursive types and union type

[eagmon]

I want types to be able to reference themselves:

{
 '_type': 'self',
 'one': 'int',
 'two': 'self',
}

We also need a union type:

{
 '_type': 'union[int, dict]'
}

Combined, these should support a type such as this:

{
 _type: foursquare,
 00: union[bool,foursquare],
 01: union[bool,foursquare],
 10: union[bool,foursquare],
 11: union[bool,foursquare],
 _default: {00: False, 01: False, 10: False, 11: False}
}