Dedicated choice types

[SimonCockx]

See part 3 of #747 (comment).

3. Support dedicated choice types (remove syntactic sugar), add support for switch expressions, support accessing common attributes with ->, and support (de)serialisation with @type (except for basic types).

A couple of things change at this point.

1. A basket can now directly be passed to a function expecting an asset, instead of wrapping it in Asset { basket: basket, ... }. The Asset {...} constructor syntax disappears.
2. Accessing common attributes in choice types changes from ->> to ->. For one-of types, the ->> syntax stays the same.
3. Discriminating a choice type now must happen by using a switch. All checks of the form if asset -> basket exists then will need to be refactored. E.g.,

   asset switch
     Basket then <item is now of type Basket>,
     Loan then <item is now of type Loan>,
     Security then <item is now of type Security>,
     default then <optional default case>
   

   Nested choice types are flattened out.
4. Serialisation becomes less nested. E.g., what used to be

   {
     "Basket": {
       "identifier": "abc123"
       "basketAttribute": 42
     }
   }
   

   now becomes

   {
     "@type": "Basket",
     "identifier": "abc123",
     "basketAttribute": 42
   }
   

Note that this can only be done after the migration to Translate 2.0.