de/serialization and validation

Each type needs a deserializer. Most types have a constructor (e.g. `int`, `float`, `bool`), and that is enough. Clients should be able to provide alternatives, though, because a deserializer also encompasses validation.

Some clients may want to serialize configurations. Most types have a `__str__` method, and that is enough. Clients should be able to provide alternatives, though, in case `__str__` does not exist or match the deserializer.

I saw the issue on Confit templates, and the idea is very similar. Going back to my assumptions (#7), though, I think it needs to be mandatory, not optional. A simple dict will be enough to define both types and defaults, however, so it should not be burdensome.