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.