Merge branch 'head_prim_fill_reducible' into 'master'

Add `head_prim_fill_reducible`.

See merge request !293
6 jobs for master in 14 minutes and 30 seconds (queued for 1 second)
Status Name Job ID Coverage
  Build
passed build-coq.8.10.dev #36985
fp

00:13:15

passed build-coq.8.7.2 #36989
fp

00:06:05

passed build-coq.8.8.2 #36988
fp

00:06:38

passed build-coq.8.9.0 #36987
fp-timing

00:05:59

passed build-coq.8.9.1 #36986
fp

00:07:19

passed build-coq.dev #36984
fp

00:14:30