Introduces a `Symbol` struct to represent identifiers, incorporating
macro hygiene context. Updates various parts of the AST compiler and
environment to use `Symbol` instead of raw `Rc<str>` for identifiers,
improving robustness for macro expansions.
Also includes:
- Adds a new example `macro_hygiene.myc`.
- Updates `.gitignore`.
- Refactors `MacroExpander` to use `ExpansionState` for cleaner template
expansion.
- Adjusts `VM::eval_observed` to ensure `after_eval` is called
correctly.
- Resets `Environment` for each test run and compilation/dumping in
`main.rs`.
Michael Schimmel