dir on a struct instance does not return all public APIs

[timkpaine]

related to #82

from csp import Struct

class Test(Struct):
    x: int

dir(Test())  # ['x']

Missing at least:

• all_fields_set
• clear
• collectts
• copy
• copy_from
• deepcopy
• deepcopy_from
• metadata (note: classmethod)
• to_dict
• to_json
• to_yaml
• update
• update_from
• And possible more, like dunders for __metadata__, __annotations__, __defaults__, __full_metadata__, __full_metadata_typed__