Grouping statement enum by macro-categories

At this time, the statements enum is massive and includes all SQL statements. What if we were to create sub-enum categories, such as DML, DDL DQL, DCL, TCL, SCL, PSM and so on?

This would clearly break APIs, but it would make much more manageable to work with statements, expecially in the cases where only a subset of the statements are desidered.

Luca

首先定位 statements enum，并审查其公共 API 及其使用情况。定义拟议的 SQL 宏类别，并评估 API 破坏和兼容性影响；当类别结构和受影响的接口达成一致，并由相关 parser 测试覆盖时，工作即告完成。

由索引模型根据 Issue 内容生成。