Optimize Faust::TransformeHelperPoly::multiply(Vect) (F@x in python).

Decrease the number of copies.

======== Mini benchmark
 hinria  …  build  wrapper  python  for i in {1..10}; do python3
test_poly_cpp.py | grep speedup;done
speedup 90.75602805478192
speedup 89.09036938690689
speedup 82.56148989913501
speedup 90.64531438391626
speedup 91.66248597807245
speedup 90.10036697594744
speedup 75.61254492055296
speedup 89.96955925554978
speedup 95.22901711420815
speedup 90.92116442476765
 hinria  …  build  wrapper  python  for i in {1..10}; do python3
test_poly_cpp.py | grep speedup;done
speedup 123.0277183801693
speedup 103.32878272246883
speedup 127.7827667275423
speedup 124.11150187961708
speedup 113.07702426409236
speedup 87.57467702373462
speedup 127.40517991813421
speedup 99.10773177189199
speedup 127.26056623369436
speedup 129.75727501516909

========

https://gitlab.inria.fr/faustgrp/faust/-/snippets/704