feat! Mimalloc as the default allocator

[anakrish]

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

[Copilot]

Pull Request Overview

Enable the use of Mimalloc as the global allocator under a cargo feature and bundle the Mimalloc C library via a Rust FFI layer.

• Add mimalloc::assign_global!() to src/lib.rs under the mimalloc feature
• Introduce new mimalloc crate implementing GlobalAlloc with tests for alloc and alloc_zeroed
• Define and expose the assign_global macro and FFI bindings in mimalloc-sys

Reviewed Changes

Copilot reviewed 49 out of 56 changed files in this pull request and generated no comments.

File	Description
src/lib.rs	Call mimalloc::assign_global!() when feature = "mimalloc"
mimalloc/src/mimalloc.rs	Implement GlobalAlloc in Mimalloc and add basic allocation tests
mimalloc/src/lib.rs	Export assign_global macro
mimalloc/mimalloc-sys/src/lib.rs	Declare extern C bindings for allocation functions

Comments suppressed due to low confidence (2)

mimalloc/src/mimalloc.rs:30

• Consider adding a test for the realloc method to verify that data is correctly preserved and the pointer is properly reallocated.

    unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {

mimalloc/src/lib.rs:5

• [nitpick] The mimalloc module duplicates the crate name, which may be confusing; consider renaming the module to something more descriptive like allocator or global.

pub mod mimalloc;

[dpokluda]