feat! Mimalloc as the default allocator

[anakrish]

Using a custom allocator like Mimalloc brings upto 30% speed improvements
for many workloads.

The code and approach (vendored) for using mimalloc is borrowed from
QSharp (https://github.com/microsoft/qsharp/tree/main/source/allocator).

Also added ACI benchmark to measure evaluation times.

[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;