Introduce DecodableSegment and remove Tuple in iterators

[wendigo]

This makes the API much easier to consume in the segments cursor style:

cur = conn.cursor('segment')
cur.execute(sql)
segments = cur.fetchall()
total_row_count = 0
row_mapper = RowMapperFactory().create(columns=cur._query.columns, legacy_primitive_types=False)
for segment in segments:
  rows = list(SegmentIterator(segment, row_mapper))
  print ("rows length is " + str(len(rows)) + " " + segment.encoding)
  total_row_count += len(rows)
print(total_row_count)

This will work the same:

segments = cur.fetchall()
total_row_count = 0
row_mapper = RowMapperFactory().create(columns=cur._query.columns, legacy_primitive_types=False)

rows = list(SegmentIterator(segments, row_mapper))
print ("rows length is " + str(len(rows)))
total_row_count += len(rows)

print(total_row_count)

[wendigo]

@mdesmet I've dropped the Tuple and the API feels more natural now

[wendigo]

@hashhar ptal, alas it's a breaking change but it was broken API from the beginning and since it was in a single release so far, I think we should just fix it and move along

[wendigo]

@hashhar can we release new version?