Generate code for handling collection and object attributes, with or without associated external type (#82)

* Altering method signature for AttrType (#78)

* Initial pass at handling plain collection types in nested object value to object value template (#78)

* Implementing CollectionType to return element type and collection type value function as strings when collection type does not have an associated external type (#78)

* Implementing AttrType and AttrValue methods on list, map, and set attributes (#78)

* Removing unneeded case statements from AttrTypes and AttrValues methods (#78)

* Implementing to/from methods for list, map, and set (#78)

* Adding a few tests (#78)

* Include elem type in embedded type for collection types AttrType() method (#78)

* Handle object attribute with or without associated external type (#78)

* Removing unused switch statement conditions (#78)

Author: bendbennett