Fix Array.append with small arrays
Fix Array.append with small arrays

Reshuffling in RRB-Trees is only reasonable when appending arrays higher
than leaf level. That is why calcToRemove and shuffle are only
implemented for those arrays. When appeding two leafes we can just give
them a parent node.

Fixes #806

JoeyEremondi/elm-pattern-effects,5outh/Elm,Axure/elm-compiler,mgold/Elm,laszlopandy/elm-compiler,pairyo/elm-compiler