Refactor: Use Symbol for identifiers and macro hygiene

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`.

examples/macro_hygiene.myc

@@ -0,0 +1,7 @@
+;; Output: 5
+(do
+(def y 1)
+(macro m1 [x] `(do (def y 10) (assign y 4)))
+(m1 8)
+(+ y (m1 8))
+)