Clean up Serializable.v somewhat

3 jobs for master in 6 minutes and 38 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #93258
au
coq:8.8

00:02:59

passed #93259
au
coq:8.9

00:02:56

passed #93260
au
coq:dev

00:03:39