Code review polishing tweaks

Generalise fully 'GeneralisedNgramsTree' and add more commentary.
4 jobs for adinapoli/issue-342 in 94 minutes (queued for 4 seconds)
latest
Status Job ID Name Coverage
  Cabal
passed #18893
cabal

00:39:52

 
  Stack
passed #18894
stack

00:06:40

 
  Bench
manual #18895
allowed to fail manual
bench
 
  Test
passed #18896
test

00:47:27