Summary

This PR introduces comprehensive support for C++ std::chrono types (durations and time_points) in simdjson's static reflection API,
enabling automatic JSON serialization and deserialization of temporal data.

Problem Solved

Previously, structs containing std::chrono::duration or std::chrono::time_point members could not be serialized/deserialized using
simdjson's static reflection features. This limitation forced users to manually convert temporal data or create custom serialization
logic.

Implementation Details

1. Serialization Support (include/simdjson/generic/ondemand/json_builder.h)

• Added template specializations for std::chrono::duration<Rep, Period>
• Added template specializations for std::chrono::time_point<Clock, Duration>
• Both types serialize to milliseconds as JSON integers for precision and compatibility

2. Deserialization Support (include/simdjson/generic/ondemand/std_deserialize.h)

• Added tag_invoke overloads for deserializing durations from milliseconds
• Added tag_invoke overloads for deserializing time_points from milliseconds since epoch
• Properly handles conversion between different duration units and clock types

3. Template Instantiation Fix (include/simdjson/generic/ondemand/json_string_builder-inl.h)

• Fixed digit_count() function to explicitly cast to uint32_t or uint64_t
• Resolves compilation errors when serializing chrono types with various integer representations

4. Comprehensive Testing (tests/builder/static_reflection_chrono_test.cpp)

• Round-trip tests for structs containing time_point members
• Tests for multiple duration types (milliseconds, seconds, minutes)
• Validation of different clock types (system_clock, steady_clock)
• Ensures millisecond precision is maintained through serialization

Key Design Decisions

• Milliseconds as standard unit: Provides good balance between precision and JSON number range
• Integer representation: More efficient and portable than ISO 8601 strings
• Non-intrusive design: No modifications needed to user structs
• Template-based approach: Works automatically with any chrono duration/time_point type

Example Usage

  struct Meeting {
      std::string title;
      std::chrono::system_clock::time_point start_time;
      std::chrono::minutes duration;
  };

  // Automatically serializes to:
  // {"title":"Team Standup","start_time":1736951234567,"duration":30000}

  Meeting m = doc.get<Meeting>(); // Deserializes seamlessly

Testing

All tests pass with reflection-enabled clang compiler. The implementation maintains exact millisecond precision through round-trip
serialization.