Support struct and union feature

[leewei05]

• parse struct and union Parse struct and union #148
• struct and union type in scope Add declaration type in scope stack #155
• struct and union initialization Implement initializer for array, struct and union #158
• struct and union access (postfix expression) Postfix record member access expression #163
• struct and union code generation
  • simple initialization Record initialization with no designators #165
  • initialization with designators Struct and Union designator initialization #177
  • pointer to struct or union
  • pass struct and union as parameters
• record type and variable declaration together as variable Parse record declaration while declaring variable #178
• nested struct and union
  • parsing and type check
  • code generation