Add a rule to alloocate an invariant and open it

Fixes #59
Proof entirely by @janno
2 jobs for master in 10 minutes and 39 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #2232
coq
iris-coq8.5.3

00:05:51

passed #2233
coq
iris-coq8.6

00:04:47