Give model and swhid objects a nicer repr()

1. hashes are now repr()ed as hash_to_bytes("1234...") instead of b"\x12\x34..."`
2. SWHID objects are now repr()ed as CoreSWHID.from_string('swh:1:...:1234...') instead of CoreSWHID(scheme='swh', version='1', object_type=..., object_id=b'\x12\x34')
3. enums are now repr()ed as MyEnum.NAME instead of "<MyEnum.NAME: 'value'>`

Thanks to these three changes, using repr() on a model object now prints
a string that can be pasted directly in a .py file to write a new
test case.