MySQL 8.0 Labs: Refactoring and Improving the Parser

In 8.0 we have introduced another bunch of parser refactoring worklogs:

WL#8067 (me): “Refactoring of the CREATE TABLE statement” with its subtasks:
- WL#8434: Refactoring of partitioning-related stuff.
- WL#8435: Cleanup and refactoring column definition stuff.
- WL#8433: Separate DD commands from regular SQL queries in the parser grammar.