define bv_to_bvn directly as the constructor

2 jobs for !250 with msammler/bitvector in 4 minutes and 34 seconds (queued for 2 seconds)
detached
Status Name Job ID Coverage
  Build
passed build-coq.8.10.2 #159942
fp

00:04:34

passed build-coq.8.13.2-mr #159941
fp

00:04:08