Column without a name produces an invalid serialization

[braingram]

import asdf
import asdf.table
asdf.dumps({"c": astropy.table.Column(data=[1, 2])})

produces a ValidationError:

ValidationError: None is not of type 'string'

Failed validating 'type' in schema['properties']['name']:
    {'description': 'The name of the column.  Each name in a\n'
                    '[table](http://stsci.edu/schemas/asdf/table/table-1.2.0) '
                    'must be\n'
                    'unique.\n',
     'pattern': '[A-Za-z_][A-Za-z0-9_]*',
     'type': 'string'}

On instance['name']:
    None

I propose that we introduce a new astropy column tag/schema and stop writing files with the core column tag/schema (but continue to read them).