feat! Mimalloc as the default allocator (#434)

This change integrates mimalloc as the default memory allocator for Regorus,
delivering significant performance improvements across all evaluation modes
and language bindings.

Technical Implementation:
- Build mimalloc in vendored mode from C sources (following QSharp approach)
- Implement GlobalAlloc trait for seamless Rust integration
- Add optional 'mimalloc' feature flag for conditional compilation
- Add comprehensive ACI benchmarks to measure evaluation performance

Performance Impact:

Rust Engine Evaluation:
- Single-threaded: ~29% improvement (423 vs 328 Kelem/s)
- Multi-threaded: Better scaling with reduced thread contention
- Fresh engines: ~24% improvement (56 vs 45 Kelem/s)

Rust Compiled Policy Evaluation:
- Single-threaded: ~41% improvement (426 vs 303 Kelem/s)
- Multi-threaded: Improved allocation efficiency under contention
- Fresh compilation: ~26% improvement (53 vs 42 Kelem/s)

C# FFI Bindings:
- Engine evaluation: ~27% improvement (279 vs 219 Kelem/s)
- Compiled policies: ~29% improvement (273 vs 211 Kelem/s)
- Better threading characteristics through improved underlying allocation

Key Benefits:
- Reduced allocation-related contention in multi-threaded scenarios
- More consistent performance across different thread counts
- Improved memory allocation efficiency for both native Rust and FFI workloads
- Better scaling characteristics for production deployments

The mimalloc integration provides substantial performance gains while
maintaining full compatibility with existing code through feature flags.

Reference: QSharp allocator implementation
(https://github.com/microsoft/qsharp/tree/main/source/allocator)

Fixes #297

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

benches/regorus_benchmark.rs

@@ -141,11 +141,46 @@ fn clone(c: &mut Criterion) {
     });
 }
 
+fn aci_policy_eval(c: &mut Criterion) {
+    let mut group = c.benchmark_group("ACI Policy Eval");
+    let rules = ["data.policy.mount_overlay", "data.policy.mount_device"];
+    for rule in rules {
+        group.bench_with_input(BenchmarkId::new("rule", rule), &rule, |b, rule| {
+            let mut engine = Engine::new();
+            engine.set_rego_v0(true);
+
+            engine
+                .add_policy_from_file("tests/aci/api.rego")
+                .expect("failed to add api.rego");
+            engine
+                .add_policy_from_file("tests/aci/framework.rego")
+                .expect("failed to add framework.rego");
+            engine
+                .add_policy_from_file("tests/aci/policy.rego")
+                .expect("failed to add policy.rego");
+            engine
+                .add_data(
+                    Value::from_json_file("tests/aci/data.json").expect("failed to load data.json"),
+                )
+                .expect("failed to add data");
+            let input =
+                Value::from_json_file("tests/aci/input.json").expect("failed to load input.json");
+            engine.set_input(input.clone());
+            engine.eval_rule(rule.to_string()).unwrap();
+            b.iter(|| {
+                engine.eval_rule(rule.to_string()).unwrap();
+            })
+        });
+    }
+    group.finish();
+}
+
 criterion_group!(
     benches,
     allow_with_simple_equality,
     allow_with_simple_membership,
-    clone
+    clone,
+    aci_policy_eval
 );
 
 criterion_main!(benches);