Add enhanced REPL with rustyline

REPL improvements:
- Tab completion for keywords, commands, and user definitions
- Persistent history saved to ~/.lux_history
- Better line editing with Emacs keybindings
- Ctrl-C to cancel input, Ctrl-D to exit
- History search with Ctrl-R

New commands:
- :info <name> - Show type information for a binding
- :env - List user-defined bindings with types

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

src/typechecker.rs

@@ -114,6 +114,11 @@ impl TypeChecker {
         }
     }
 
+    /// Look up a type scheme by name (for REPL :info command)
+    pub fn lookup(&self, name: &str) -> Option<&TypeScheme> {
+        self.env.bindings.get(name)
+    }
+
     /// Type check a program
     pub fn check_program(&mut self, program: &Program) -> Result<(), Vec<TypeError>> {
         // First pass: collect all declarations