refactor: switch containers to mimalloc-aware aliases and use sel_t for row indices

[liulx20]

This pull request introduces a new container abstraction to standardize and optimize memory allocation for column data structures, with support for mimalloc-based allocation when enabled. It replaces standard STL containers with custom types throughout the codebase, updates function signatures to use the new types, and ensures consistent usage in edge and context column classes. Additionally, it adds a compile-time flag for mimalloc integration.

Container abstraction and allocator support:

• Added container_types.h defining neug::vector, neug::flat_hash_map, neug::flat_hash_set, and allocator selection based on the NEUG_WITH_MIMALLOC macro. Also introduced sel_t and sel_vec_t as selection index types.
• Updated CMakeLists.txt to add the NEUG_WITH_MIMALLOC compile definition and changed mimalloc override settings for more controlled allocator usage.

These changes improve memory management flexibility, enable allocator customization, and standardize container usage across the codebase.

Fixes