Empty case split should graft into FunBinds matches if possible

[isovector]

Given (>>=) x f = case x of, running Complete code constructors today produces

  (>>=) x f = case x of
    Goto s -> _
    Terminal a -> _
    Transition g -> _
    Invoke s mx g -> _

but instead could produce

  (>>=) (Goto s) f = _
  (>>=) (Terminal a) f = _
  -- etc...

[Ailrun]

As a note, it should create FunBinds only when other variables are not patterns, because if they are, FunBinds may change the evaluation order of arguments.