Bugfix for BinaryEncoder positions

[gsilvestrin]

When merging multiple arrays, BinaryEncoder creates a array of positions/offsets that it is too big.

For instance in the following array:

&[
    &StringArray::from(vec![Some("a"), None, Some("cd"), None]),
    &StringArray::from(vec![Some("f"), None, Some("gh"), None]),
    &StringArray::from(vec![Some("t"), None, Some("uv"), None]),
]

Each sub-array has its own offset array. Since each array has 4 elements the offset array has 5 elements

offsets: [0, 1, 1, 3, 3]
offsets: [0, 1, 1, 3, 3]
offsets: [0, 1, 1, 3, 3]

When BinaryEncoder merge all sub-arrays into a single Array, it creates an array of length 12 (4*3) but an offset array with 15 elements

pos_array: PrimitiveArray<Int64>
[
  4,
  5,
  5,
  7,
  7,
  8,
  9,
  9,
  11,
  11,
  12,
  13,
  13,
  15,
  15,
]

This PR makes it create the correct offset array with 13 elements

pos_array: PrimitiveArray<Int64>
[
  4,
  5,
  5,
  7,
  7,
  8,
  8,
  10,
  10,
  11,
  11,
  13,
  13,
]

[eddyxu]

could it be simpler like

let pos_builder = Builder::new(...)
pos_builder.append(start_pos);

for array in arrs.iter() {
    let offsets = array.offsets_array();
    let last_offset = pos_builder.last() # somehow
    pos_builder.append_iter(offsets.iter().skip(1).map(|p| p + last))
}

?

[changhiskhan]

there's no last but maybe pos_builder.values_slice()[pos_builder.len() - 1] would work?

[gsilvestrin]

Moved to this issue: #700