Replace type.Initializiation with a type constructor function

[Botffy]

Currently the interpreter uses the Type of a variable to determine how to initialize it, types having an InitializationMethod enum value.

It'd be more flexible, and better for the separation of concerns, if the Grammar could define a SpecialName.TYPE_CONSTRUCTOR nullary function for types, and the Interpreter would call that on initialization. These would be implemented as instructions, at least initially.