Memory refactor

[Andy-Jost]

Major refactoring of the memory package.

Overview
This PR refactors the _memory.pyx module into a dedicated package (_memory/) to address its growing size and complexity, which were hindering further development. The primary goals are to physically separate the code into more manageable submodules, simplify the internal logic, and enhance the overall structure, including the addition of .pxd headers for better Cython integration.

Major Changes

• Split _memory.pyx into submodules, the major ones being the following:
  • Buffers: _buffer.*
  • Device memory resources: _dmr.*
  • IPC (Inter-Process Communication): _ipc.*
  • Virtual memory management: _vmm.*
• Introduced Cython headers (.pxd) for public definitions to improve modularity and type safety.
• Refactored DeviceMemoryResource to isolate IPC-related code, reducing coupling.
• Simplified IPC implementation by adding an IPCData class to encapsulate relevant data members and eliminating a redundant uuid field.
• Streamlined the class hierarchy by removing unnecessary classes.
• Simplified the Cython interface for memory allocation and deallocation operations.

Minor Improvements

• Added __all__ lists to modules for explicit control over exports.
• Extracted long implementation functions from class definitions to make classes more concise and readable.
• Renamed various private attributes and methods for consistency (e.g., _handle instead of _mempool_handle).
• Consolidated and alphabetized property definitions for better organization.
• Converted additional classes and functions to Cython for performance gains.

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[Andy-Jost]

/ok to test cf4dc9d

[Andy-Jost]

/ok to test 19e4b8f

[rparolin]

Generally looks good to me. I left some general comments that I consider non-blocking.

@leofang Any major concerns you'd like to see address before this gets merged?

[Andy-Jost]

/ok to test ff3820f

[leofang]

Style: Would be better to call out what's being imported. Maintaining __all__ or having to chasing after each module for their __all__ is not fun. I don't recall we ever maintain __all__ in any other modules.

[Andy-Jost]

I guess I have the opposite preference. E.g., if I add something to, say, _buffer I find it easier to update the __all__ list there rather than in a separate file.

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