feat: implement Cranelift JIT compiler for native code execution

Add a JIT compiler using Cranelift that compiles Lux functions to native
machine code. Achieves ~160x speedup over the tree-walking interpreter
(fib(30): 11.59ms JIT vs 1.87s interpreter).

Supports: arithmetic, comparisons, conditionals, let bindings, function
calls, and recursion. Compile time overhead is minimal (~500µs).

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

src/main.rs

@@ -1,6 +1,7 @@
 //! Lux - A functional programming language with first-class effects
 
 mod ast;
+mod compiler;
 mod debugger;
 mod diagnostics;
 mod exhaustiveness;