Make `iPure` non-local so it can be used for defining custom `iDestruct`-like tactics.

6 jobs for master in 14 minutes and 4 seconds
Status Name Job ID Coverage
  Build
passed build-coq.8.10.dev #35970
fp

00:12:46

passed build-coq.8.7.2 #35974
fp

00:05:36

passed build-coq.8.8.2 #35973
fp

00:06:41

passed build-coq.8.9.0 #35972
fp-timing

00:05:49

passed build-coq.8.9.1 #35971
fp

00:07:07

passed build-coq.dev #35969
fp

00:14:04