Added support for std::span; resolves #457

[liuzicheng1987]

No description provided.

[gemini-code-assist]

Summary of Changes

Hello @liuzicheng1987, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances reflect-cpp by introducing robust support for std::span, enabling its seamless serialization and deserialization. Alongside this new feature, I've clarified the necessary memory management practices for std::span and std::string_view when using the rfl::AllowRawPtrs processor, ensuring users are fully aware of their responsibilities.

Highlights

• New Container Support: I've added full read and write support for std::span types within the reflect-cpp library.
• Documentation Updates: I've updated README.md to list std::span and std::string_view as newly supported containers. Crucially, docs/concepts/processors.md now includes detailed warnings and cleanup instructions for std::span and std::string_view when rfl::AllowRawPtrs is used, emphasizing the user's responsibility for memory management.
• Memory Management Clarification: The library now explicitly warns users about the manual memory deallocation required for std::span and std::string_view when reading with rfl::AllowRawPtrs, aligning their behavior and warnings with raw pointers.
• New Test Coverage: A dedicated test case has been added to validate std::span serialization and deserialization functionality with the JSON backend.

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[gemini-code-assist]

Code Review

This pull request adds support for std::span, including serialization, deserialization, and documentation updates. There are memory safety issues in the std::span parser and a memory leak in the test case.