Condition could have more detailed fields

Currently, when I parse a query, I get WHERE as Condition instances. These have an expr field, which is just a string and not very useful when analyzing a query. It would be really nice if we could be able to get more detail from that, like the operator, operands, etc.

Examples:

// c.id = 2
[
    leftOperand: ColumnReference [ column: 'id', tableAlias: 'c' ] ,
    operator: Equals []
    rightOperand: Constant [ value: 2 ]
]

// customer.address_id IS NOT NULL
[
    leftOperand: ColumnReference [ column: 'address_id', tableName: 'customer' ] ,
    operator: IsNotNull
    rightOperand: null
]

首先定位 parser 的 Condition 实例和现有的 expr 字段。比较所要求的 c.id = 2 和 customer.address_id IS NOT NULL 示例，然后定义必须公开哪些结构化的 operator 和 operand 信息；当 conditions 提供这些详细信息而不只是一个字符串时，工作就完成了。