This pull request introduces a parser and an Abstract Syntax Tree (AST) to the project, enabling the transformation of tokens into structured representations. The changes include the addition of a new Parser module, the implementation of an AST in src/ast.rs, and updates to the main program to integrate these components.

Parser and AST Implementation:

• Added src/ast.rs to define the AST structure, including Program, Statement, and Expression types, as well as support for constructs like LetStatement, WhenStatement, FunctionDef, and BinaryOperation.
• Introduced the Parser module in src/parser.rs, which parses tokens into an AST. It includes methods for handling statements (parse_statement) and expressions (parse_expression) with support for constructs like let, show, when, and function definitions.

Integration into the Main Program:

• Updated src/main.rs to include the parser and ast modules and integrate the parsing step into the main execution flow. This includes creating a Parser instance, generating the AST, and printing it for debugging purposes. [1] [2]