Add support for Data objects with ivarsHEADmaster

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.

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