`Reader::iter` may return vectors with the wrong dimensions

[dureuill]

When:

• The dimensions is not a multiple of 64
• The distance is binary quantized (tested with BinaryQuantizedCosine, might reproduce with other distances)
• Then Reader::iter returns vectors with a dimension rounded up to the next multiple of 64.

User workaround:

• truncate any obtained vector to the dimensions of the reader.

Note: hannoy has the same issue: nnethercott/hannoy#78