Implement pointer support, address-of/dereference operators, and pointer assignments

[hand-burger]

Summary

This PR adds support for pointers in the RCC compiler. It introduces pointer types (e.g., int* , int** ), unary address-of ( & ) and dereference ( * ) operators, and pointer dereference assignments (e.g., *p = 100; ).

Changes

1. Lexer & AST:

• Added tokenization support for the ampersand ( & ) as Token::Ampersand .
• Updated the AST to include a recursive Type::Pointer(Box) variant and equipped unary operators with Deref and AddrOf modes.

2. Parser:

• Implemented type parsing supporting multiple levels of pointer qualifiers (e.g., int* , int** ).
• Updated functions to support pointer return types.
• Refactored variable declarations to store their types, and assignments to parse their left-hand side targets as unary expressions.

3. LLVM Code Generation:

• Migrated the symbol table to store variable type information (Type, PointerValue) to properly compile LLVM load and store instructions under LLVM 17's opaque pointers architecture.
• Implemented type_of_expr to statically determine expression types.
• Implemented emit_lvalue to locate memory targets of variables and pointer dereferencing expressions.
• Refactored emit_expr to return BasicValueEnum supporting both integers and pointer types.

4. Tests:

• Added unit test cases for the parser and codegen.
• Added integration test cases verifying basic pointers, dereference assignments, and multi-level pointers ( int** pp ).