Introducing enum for Operators in Where method

[roberto-butti]

With where() method you can define the operator == != etc for filtering elements.
I would like to introduce enum for listing the operators, something like:

enum Operator: string {
    case EQUAL = '==';
    case GREATER_THAN = '>';
    case LESS_THAN = '<';
    case GREATER_THAN_OR_EQUAL = '>=';
    case LESS_THAN_OR_EQUAL = '<=';
    case NOT_EQUAL = '!=';
    case STRICT_NOT_EQUAL = '!==';
    case IN = 'in';
    case HAS = 'has';
}

[roberto-butti]

closed by #31