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>

Cargo.toml

@@ -15,6 +15,13 @@ serde_json = "1"
 rand = "0.8"
 reqwest = { version = "0.11", features = ["blocking", "json"] }
 
+# Cranelift for native compilation
+cranelift-codegen = "0.95"
+cranelift-frontend = "0.95"
+cranelift-module = "0.95"
+cranelift-jit = "0.95"
+target-lexicon = "0.12"
+
 [dev-dependencies]
 tempfile = "3"