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/aci_benchmark.rs

@@ -0,0 +1,80 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+use regorus::{Engine, Value};
+
+use criterion::{criterion_group, criterion_main, BenchmarkId, Criterion};
+use serde::{Deserialize, Serialize};
+use walkdir::WalkDir;
+
+use std::path::Path;
+
+#[derive(Serialize, Deserialize, PartialEq, Debug)]
+struct TestCase {
+    note: String,
+    data: Value,
+    input: Value,
+    modules: Vec<String>,
+    query: String,
+    want_result: Value,
+}
+
+#[derive(Serialize, Deserialize, PartialEq, Debug)]
+struct YamlTest {
+    cases: Vec<TestCase>,
+}
+
+fn aci_policy_eval(c: &mut Criterion) {
+    let dir = Path::new("tests/aci");
+    for entry in WalkDir::new(dir)
+        .sort_by_file_name()
+        .into_iter()
+        .filter_map(|e| e.ok())
+    {
+        let path = entry.path();
+        if !path.to_string_lossy().ends_with(".yaml") {
+            continue;
+        }
+
+        let yaml = std::fs::read(path).expect("failed to read yaml test");
+        let yaml = String::from_utf8_lossy(&yaml);
+        let test: YamlTest = serde_yaml::from_str(&yaml).expect("failed to deserialize yaml test");
+
+        for case in &test.cases {
+            let rule = case.query.replace("=x", "");
+            c.bench_with_input(
+                BenchmarkId::new("case ", format!("{} {}", &case.note, &rule)),
+                &case,
+                |b, case| {
+                    let mut engine = Engine::new();
+                    engine.set_rego_v0(true);
+
+                    engine
+                        .add_data(case.data.clone())
+                        .expect("failed to add data");
+                    engine.set_input(case.input.clone());
+
+                    for (idx, rego) in case.modules.iter().enumerate() {
+                        if rego.ends_with(".rego") {
+                            let path = dir.join(rego);
+                            let path = path.to_str().expect("not a valid path");
+                            engine
+                                .add_policy_from_file(path)
+                                .expect("failed to add policy");
+                        } else {
+                            engine
+                                .add_policy(format!("rego{idx}.rego"), rego.clone())
+                                .expect("failed to add policy");
+                        }
+                    }
+
+                    b.iter(|| {
+                        engine.eval_rule(rule.clone()).unwrap();
+                    })
+                },
+            );
+        }
+    }
+}
+
+criterion_group!(aci_benches, aci_policy_eval);
+criterion_main!(aci_benches);