insertLet and insertPat now handle both up/down
insertLet and insertPat now handle both up/down

- pluck now returns a function to rebuild residual let expression
- with this, up/down cases for insertLet and insertPat are now
  handled in a uniform, single-pass way

ravichugh/sketch-n-sketch,ravichugh/sketch-n-sketch,ravichugh/sketch-n-sketch,ravichugh/sketch-n-sketch,ravichugh/sketch-n-sketch,ravichugh/sketch-n-sketch