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>

flake.nix

@@ -22,6 +22,8 @@
             rustToolchain
             cargo-watch
             cargo-edit
+            pkg-config
+            openssl
           ];
 
           RUST_BACKTRACE = "1";