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>

Cargo.toml

@@ -32,6 +32,7 @@ http = []
 glob = ["dep:globset"]
 graph = []
 jsonschema = ["dep:jsonschema"]
+mimalloc = ["dep:mimalloc"]
 net = []
 no_std = ["lazy_static/spin_no_std"]
 opa-runtime = []
@@ -51,6 +52,7 @@ full-opa = [
     "hex",
     "http",
     "jsonschema",
+    "mimalloc",
     "net",
     "opa-runtime",
     "regex",
@@ -114,6 +116,7 @@ rand = { version = "0.9.0", default-features = false, features = ["thread_rng"],
 # Causes the project to link with the Spectre-mitigated CRT and libs.
 msvc_spectre_libs = { version = "0.1", features = ["error"], optional = true }
 dashmap = { version = "6.1", default-features = false, optional = true }
+mimalloc = { path = "mimalloc", optional = true }
 
 [dev-dependencies]
 anyhow = "1.0.45"
@@ -170,6 +173,10 @@ name = "compiled_policy_evaluation_benchmark"
 path = "benches/evaluation/compiled_policy_evaluation_benchmark.rs"
 harness = false
 
+[[bench]]
+name = "aci_benchmark"
+harness = false
+
 [[example]]
 name="regorus"
 harness=false