Parse struct and union

[leewei05]

This PR parses struct and union type declaration.

• A new declaration node RecordDeclNode for struct and union.
• A new declaration node FieldNode for fields of struct or union.
• Introduces StructType and UnionType.

TODO: Just like we discusses, I will create a new symbol table (probably the same members but different name) under scope in the future PRs.

The output format is copied from clang:

$ clang -Xclang -ast-dump test/typecheck/struct.c
-FunctionDecl 0x55bf9d309910 <test/typecheck/struct.c:1:1, line:17:1> line:1:5 main 'int ()'
  `-CompoundStmt 0x55bf9d309ed0 <col:12, line:17:1>
    |-DeclStmt 0x55bf9d309aa0 <line:2:3, col:12>
    | `-RecordDecl 0x55bf9d3099f8 <col:3, col:10> col:10 struct ss
    |-DeclStmt 0x55bf9d309c98 <line:4:3, line:8:4>
    | `-RecordDecl 0x55bf9d309ab8 <line:4:3, line:8:3> line:4:10 struct birth definition
    |   |-FieldDecl 0x55bf9d309b78 <line:5:5, col:9> col:9 date 'int'
    |   |-FieldDecl 0x55bf9d309be0 <line:6:5, col:9> col:9 month 'int'
    |   `-FieldDecl 0x55bf9d309c48 <line:7:5, col:9> col:9 year 'int'
    |-DeclStmt 0x55bf9d309e88 <line:10:3, line:14:4>
    | `-RecordDecl 0x55bf9d309cb0 <line:10:3, line:14:3> line:10:3 struct definition
    |   |-FieldDecl 0x55bf9d309d68 <line:11:5, col:9> col:9 quarter 'int'
    |   |-FieldDecl 0x55bf9d309dd0 <line:12:5, col:9> col:9 dime 'int'
    |   `-FieldDecl 0x55bf9d309e38 <line:13:5, col:9> col:9 penny 'int'
    `-ReturnStmt 0x55bf9d309ec0 <line:16:3, col:10>
      `-IntegerLiteral 0x55bf9d309ea0 <col:10> 'int' 0

Related to: #140

[Lai-YT]

Everything else looks good to me. 🥳

[Lai-YT]

LGTM! Looking forward to the support of struct and union! 🚀