[ngrams] MVar -> TVar refactoring

Also, some fixes to NodeStory (it inserts ngrams when saving node
story, when they don't exist).
4 jobs for 281-dev-ngrams-fixes in 7 minutes and 40 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Stack
passed #10313
stack

00:04:35

 
  Cabal
canceled #10314
cabal

00:03:05

 
  Bench
canceled #10315
bench

 
  Test
canceled #10316
test