get names() {
        const _i = this._inner;
        if (!_i.names) {
            _i.names = this.columns.map(c => c.escapedName).join();
        }
        return _i.names;
    }
@name helpers.ColumnSet#names
@type string
@readonly
@description
Returns a string - comma-separated list of all column names, properly escaped.

@example
const cs = new ColumnSet(['id^', {name: 'cells', cast: 'int[]'}, 'doc:json']);
console.log(cs.names);
//=> "id","cells","doc"