Parallelize integer dot product
The following is not parallel

https://github.com/mratsim/Arraymancer/blob/a4b79c86c184caceb28e1ed506012babd03f8cd4/src/tensor/operators_blas_l1.nim#L31-L36

Pending [Laser](https://github.com/numforge/laser) and https://github.com/numforge/laser/pull/4