Add full support for user-defined generic types and functions:
- Add type_params field to TypeChecker to track type parameters in scope
- Update resolve_type() to resolve type parameters to their bound variables
- Update function_type() to bind type parameters and return polymorphic TypeScheme
- Update type declaration handling for generic ADTs (e.g., Pair<A, B>)
Generic functions and types now work:
fn identity<T>(x: T): T = x
type Pair<A, B> = | MkPair(A, B)
fn first<A, B>(p: Pair<A, B>): A = ...
Add examples/generics.lux demonstrating:
- Generic identity function
- Generic Pair type with first/second accessors
- Generic mapOption function
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
