Add tests for appending short arrays

Test Array.append A B where length(A) <= M and length(B) <= M, but
length(A + B) > M.
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
Merge pull request #813 from Xashili/arrayAppend

Fix Array.append