feat: add AllocT template parameter to HashMap and HashSet

[tarun-t]

Adds a defaulted AllocT template parameter to all HashMap (5/6/7/8) and HashSet (2/3/4/8), replacing hardcoded malloc/free with allocator_traits. Fixes alignment for over-aligned value types. Backwards compatible.

Closes #39

[gemini-code-assist]

Summary of Changes

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This pull request significantly enhances the emhash library by introducing customizable memory allocation for its hash map and hash set containers. By replacing direct malloc/free calls with std::allocator_traits, the library now supports user-defined allocators, which is crucial for applications requiring fine-grained control over memory, such as memory pooling or specific alignment requirements. This change also addresses and corrects memory alignment for complex data types, ensuring robustness and performance, all while maintaining backward compatibility with existing codebases.

Highlights

• Custom Allocator Support: Introduced a defaulted AllocT template parameter to all HashMap (5/6/7/8) and HashSet (2/3/4/8) implementations, allowing users to provide custom memory allocators.
• Memory Management Modernization: Replaced hardcoded malloc/free calls with std::allocator_traits for all memory allocations and deallocations, enhancing flexibility and adherence to C++ allocator conventions.
• Alignment Correction: Implemented logic to correctly handle and fix alignment for over-aligned value types within the hash containers.
• Allocator-Aware Constructors and Operators: Added new constructors and assignment operators that accept allocator arguments and correctly implement allocator propagation rules (e.g., select_on_container_copy_construction, propagate_on_container_copy_assignment).
• Comprehensive Testing: Added a new test file (test/test_allocator.cpp) to thoroughly verify the new allocator functionality, including tests for default allocators, over-aligned types, std::pmr allocators, and allocator propagation behavior.

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[gemini-code-assist]

Code Review

This pull request introduces std::allocator support across all emhash hash set and hash map containers. The changes involve updating class templates to accept an AllocT parameter, replacing raw malloc/free calls with allocator-aware alloc_bucket/dealloc_bucket methods, and modifying constructors, copy/move constructors, and assignment operators to correctly handle allocator construction, propagation, and assignment. Additionally, get_allocator() methods were added, and a new test file (test/test_allocator.cpp) was included to verify the new allocator functionality, including std::pmr allocators and their propagation behavior.