Support user-defined types for type constraint.

[linlin-s]

    /**
     * Get the value of constraint 'type' in IonType format.
     * @return the value of 'type' in IonType format.
     */
    public IonType getIonType() {
        return IonType.valueOf(getIsl().get(KEYWORD_TYPE).toString().toUpperCase());
    }

This method only work when the provided constraint type is one of core types. If the provided type is user-defined, this method won't work. And we need to look up the type definition by using method Schema#getType(String name) .

Referred comments:

• Refactoring and updating data generating methods. #34 (comment)
• Refactoring and updating data generating methods. #34 (comment)