feat: implement type classes / traits

Add support for type classes (traits) with full parsing, type checking, and
validation. The implementation includes:

- Trait declarations: trait Show { fn show(x: T): String }
- Trait implementations: impl Show for Int { fn show(x: Int) = ... }
- Super traits: trait Ord: Eq { ... }
- Trait constraints in where clauses: where T: Show + Eq
- Type parameters on traits: trait Functor<F> { ... }
- Default method implementations
- Validation of required method implementations

This provides a foundation for ad-hoc polymorphism and enables
more expressive type-safe abstractions.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/interpreter.rs

@@ -881,7 +881,7 @@ impl Interpreter {
                 Ok(value)
             }
 
-            Declaration::Effect(_) | Declaration::Type(_) => {
+            Declaration::Effect(_) | Declaration::Type(_) | Declaration::Trait(_) | Declaration::Impl(_) => {
                 // These are compile-time only
                 Ok(Value::Unit)
             }