This sets the ivars _before_ calling initialize, which feels wrong. But
Data doesn't give us any mechanism for setting the members other than 1)
initialize, or 2) drop down into the C API. Since initialize freezes
the object, we need to set the ivars before that. I think this is a
reasonable compromise—if users need better handling, they can implement
their own `encode_with` and `init_with`. But it will lead to unhappy
surprises for some users.
Alternatively, we could use the C API, similarly to Marshal. Psych _is_
already using the C API for path2class and build_exception. This would
be the least surprising behavior for users, I think.