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>

bindings/ruby/Cargo.lock

@@ -95,9 +95,9 @@ checksum = "5e764a1d40d510daf35e07be9eb06e75770908c27d411ee6c92109c9840eaaf7"
 
 [[package]]
 name = "bitflags"
-version = "2.9.2"
+version = "2.9.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "6a65b545ab31d687cff52899d4890855fec459eb6afe0da6417b8a18da87aa29"
+checksum = "34efbcccd345379ca2868b2b2c9d3782e9cc58ba87bc7d79d5b53d9c9ae6f25d"
 
 [[package]]
 name = "borrow-or-share"
@@ -129,9 +129,9 @@ checksum = "175812e0be2bccb6abe50bb8d566126198344f707e304f45c648fd8f2cc0365e"
 
 [[package]]
 name = "cc"
-version = "1.2.33"
+version = "1.2.34"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "3ee0f8803222ba5a7e2777dd72ca451868909b1ac410621b676adf07280e9b5f"
+checksum = "42bc4aea80032b7bf409b0bc7ccad88853858911b7713a8062fdc0623867bedc"
 dependencies = [
  "shlex",
 ]
@@ -254,9 +254,9 @@ dependencies = [
 
 [[package]]
 name = "form_urlencoded"
-version = "1.2.1"
+version = "1.2.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "e13624c2627564efccf4934284bdd98cbaa14e79b0b5a141218e507b3a823456"
+checksum = "cb4cb245038516f5f85277875cdaa4f7d2c9a0fa0468de06ed190163b1581fcf"
 dependencies = [
  "percent-encoding",
 ]
@@ -419,9 +419,9 @@ dependencies = [
 
 [[package]]
 name = "idna"
-version = "1.0.3"
+version = "1.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "686f825264d630750a544639377bae737628043f20d38bbc029e8f29ea968a7e"
+checksum = "3b0875f23caa03898994f6ddc501886a45c7d3d62d04d2d90788d47be1b1e4de"
 dependencies = [
  "idna_adapter",
  "smallvec",
@@ -578,6 +578,20 @@ version = "2.7.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "32a282da65faaf38286cf3be983213fcf1d2e2a58700e808f83f4ea9a4804bc0"
 
+[[package]]
+name = "mimalloc"
+version = "2.1.2"
+dependencies = [
+ "mimalloc-sys",
+]
+
+[[package]]
+name = "mimalloc-sys"
+version = "0.0.0"
+dependencies = [
+ "cc",
+]
+
 [[package]]
 name = "minimal-lexical"
 version = "0.2.1"
@@ -719,9 +733,9 @@ dependencies = [
 
 [[package]]
 name = "percent-encoding"
-version = "2.3.1"
+version = "2.3.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "e3148f5046208a5d56bcfc03053e3ca6334e51da8dfb19b6cdc8b306fae3283e"
+checksum = "9b4f627cb1b25917193a259e49bdad08f671f8d9708acfd5fe0a8c1455d87220"
 
 [[package]]
 name = "phf"
@@ -925,6 +939,7 @@ dependencies = [
  "globset",
  "jsonschema",
  "lazy_static",
+ "mimalloc",
  "msvc_spectre_libs",
  "rand",
  "regex",
@@ -1162,9 +1177,9 @@ checksum = "673aac59facbab8a9007c7f6108d11f63b603f7cabff99fabf650fea5c32b861"
 
 [[package]]
 name = "url"
-version = "2.5.4"
+version = "2.5.6"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "32f8b686cadd1473f4bd0117a5d28d36b1ade384ea9b5069a1c40aefed7fda60"
+checksum = "137a3c834eaf7139b73688502f3f1141a0337c5d8e4d9b536f9b8c796e26a7c4"
 dependencies = [
  "form_urlencoded",
  "idna",