Move Fin theory from vector.v to fin.v; improve inv_fin to match what inv_vec can do

2 jobs for master in 2 minutes and 17 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #2837
fp-timing
stdpp-coq8.5

00:02:17

passed #2838
fp-timing
stdpp-coq8.6

00:01:53