Return all of the columns registered for a given table.

    Parameters
    ----------
    table_name : str

    Returns
    -------
    columns : dict of column wrappers
        Keys will be column names.

def _columns_for_table(table_name):
    """
    Return all of the columns registered for a given table.

    Parameters
    ----------
    table_name : str

    Returns
    -------
    columns : dict of column wrappers
        Keys will be column names.

    """
    return {cname: col
            for (tname, cname), col in _COLUMNS.items()
            if tname == table_name}