Fix Issue #10

[anka-213]

The old code said "If the left branch is empty, then ignore the right branch", which is obviously wrong. The intended behavior was to replace the empty branch with a "Pure".

Test case:

import Control.Zipper
import Control.Lens

-- A tree with potentially empty branches
data T a = Lf a | Nil | Node (T a) (T a) deriving Show

-- A traversal for the tree
trav :: Traversal (T a) (T b) a b
trav k = go where
 go Nil        = pure Nil
 go (Lf a)     = Lf <$> k a
 go (Node a b) = Node <$> go a <*> go b

-- This tree triggers the bug.
brokenTree :: T Int
brokenTree = (Lf 1`Node`Nil) `Node` Lf 2

main =   zipper brokenTree
     &   within trav -- A zipper pointing at "Lf 1"
     >>= rightward   -- At this point, the "Lf 1" is forgotten
     <&> rezip       -- This line fails with "jacketOuts: wrong shape"
     >>= print

This is (most likely) the cause of bug #10.

[anka-213]

An alternative, way shorter test case:

zipper [Just 1, Nothing, Just 2] & fromWithin (traverse._Just) & rightmost & tug leftward & rezip