Make instances EqDecision and Countable of gmultiset less eager.

Fix fixes issue #63.
2 jobs for master in 10 minutes and 53 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #2326
coq
iris-coq8.5.3

00:05:54

passed #2327
coq
iris-coq8.6

00:04:58