Normalise tree with leaf root nodes after slicing.
Normalise tree with leaf root nodes after slicing.

Leaf root nodes are allowed to contain 1-RRB_BRANCHING elements in them
when there is no tail included. However, they are not allowed to when we
have a tail: They must be RRB_BRANCHING in size at that point. This
commit resolves this issue by doing two things:

- Merge tail + root if they both can be fitted inside a single, new
  tail.
- Otherwise transfer elements from tail to a new root, which is
  RRB_BRANCHING wide. The remaining elements from the tail is then
  copied over to a new, smaller tail.

hyPiRion/c-rrb,hyPiRion/c-rrb,hyPiRion/c-rrb,hyPiRion/c-rrb