Type families: fixes in flattening & finalisation

Type families: fixes in flattening & finalisation
* Finalisation didn't do the right thing for equalities x ~ y, where
  x was instantiated, but not zonked and y flexible (need to do y := x)
* During flattening we weren't careful enough when turning wanteds
  intermediates into locals

Both bugs showed up in a small example of SPJ:

  linear :: HasTrie (Basis v) => (Basis v, v)
  linear =  basisValue

  class HasTrie a where

  type family Basis u :: *

  basisValue :: (Basis v,v)
  basisValue = error "urk"