vx_expression_clone, remove vx_file_write_array

[myrrc]

Preparatory changes split from new C++ API PR.

• Add vx_expression_clone
• Remove accidental const modifier from vx_expression_not's output.
• Fix a memory leak in vx_struct_fields_builder_add_field where a non-UTF8 field
  throws before DType was converted from a raw pointer to an Arc.
• Make vx_array_sink_push return an error if array's dtype doesn't match sink's dtype.
• Allow passing null pointers to FFI's free() functions.
• Remove vx_file_write_array in favor of sink interface.

[codspeed-hq]

Merging this PR will improve performance by 18.17%

⚡ 3 improved benchmarks
✅ 1661 untouched benchmarks
⏩ 42 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	chunked_varbinview_opt_canonical_into[(1000, 10)]	205.7 µs	168.8 µs	+21.88%
⚡	Simulation	chunked_varbinview_into_canonical[(1000, 10)]	205.9 µs	169.5 µs	+21.49%
⚡	Simulation	eq_i64_constant	298.5 µs	267.8 µs	+11.46%

Tip

Curious why this is faster? Comment @codspeedbot explain why this is faster on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing myrrc/ffi-cxx (e784ed1) with develop (443b79b)}

Footnotes

1. 42 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩