MIR: Add minimal AST parse result

Context

This builds on the changes from !9999 (merged) and adds a minimal AST as the result of the parsing. Again, only a small subset of the language that is required to represent the Fibonacci program from !9906 (merged), is included.

The included tests are implemented by using the built-in pretty printer. The AST result from parsing is pretty printed and compared with the expected output.

Related: !9999 (merged)

Manually testing the MR

Please run cargo test command to run the included tests.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR