Collapses BitmapIndexed and Full with just one Leaf or Collision child.
Note that in delete the line:
Empty | A.length ary == 1 -> Empty
is unnecessary if we decide that the collapsing should be an invariant. That's because it would correspond to a situation where the considered BitmapIndexed had just one key-value pair at the end (and thus should have been represented as a Leaf in the first place).
Collapse nodes with one Leaf/Collision child (issue #47)