Combine strings, numbers, bools into literals

Strings, numbers and bools are set in the same way so we might as well
combine them into one list named literals

Remove Mapper.set_string(), set_number(), set_bool()
Add Mapper.set_literal()
Use Mapper.parse_map() for Person.unserialize()

Remove unserialize_service() as it shouldnt be needed anymore
Use Mapper.parse_map() for Feed.unserialize()
remove log message
Use Mapper.parse_map() for most of Image.unserialize()