fix: address clippy warnings and remove dead code in native C2R

- Add #[allow(clippy::too_many_arguments)] to write_elements_slow
- Remove unused functions that were added during development:
  - write_variable_length_to_buffer
  - get_element_size
  - try_bulk_copy_primitive_array_with_nulls
  - write_array_data_to_buffer
  - write_array_data_to_buffer_for_map
- Remove #[inline] from write_struct_to_buffer (too large/complex)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: andygrove