Reading FIXED_LEN_BYTE_ARRAY columns with nulls is inefficient

[etseidl]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
When reading a Parquet file with FIXED_LEN_BYTE_ARRAY columns with nulls present one necessary operation is moving the fixed-length data into the correct location within the output buffer to take into account null slots. This is handled by the pad_nulls function in the ValuesBuffer trait. The inner loop of this function

  for i in 0..byte_length {
      self.buffer[level_pos_bytes + i] = self.buffer[value_pos_bytes + i]
  }

works well when the fixed width is low (<= 4), but for larger widths this loop is quite inefficient.

Describe the solution you'd like
Rewriting the inner loop for longer fixed-size arrays can speed this operation up considerably. In particular, by copying slices of the buffer to another location in the buffer, the compiler can vectorize the move, e.g.

  let split = self.buffer.split_at_mut(level_pos_bytes);
  let dst = &mut split.1[..byte_length];
  let src = &split.0[value_pos_bytes..value_pos_bytes + byte_length];
  for i in 0..byte_length {
      dst[i] = src[i]
  }

Describe alternatives you've considered
I tried Vec::copy_within but it was slower than the vectorized copy.

Additional context

[alamb]

label_issue.py automatically added labels {'parquet'} from #6297