Add is_closed_term tactic

2 jobs for !345 with msammler/assert_closed in 4 minutes and 25 seconds (queued for 5 seconds)
detached
Status Name Job ID Coverage
  Build
passed build-coq.8.11.2 #161017
fp

00:04:23

passed build-coq.8.14.1-mr #161016
fp

00:04:18