API harmonization: insert

[jblachly]

Currently, AVL tree and Splay tree insert function signatures differ:

AVL:
Node *insert(Node *x, out uint cnt)

Splay:
Node * insert(IntervalType i)

In practice then calling code looks like:

                    foreach(link; c.links)
                    {
                        version(avl)    { uint cnt; (*tree).insert( new IntervalTreeNode!ChainLink(*link), cnt ); }
                        version(splay)  (*tree).insert(*link);

Proposal:
Both should take an IntervalType (current behavior of splay tree)

As a bonus, could also include overload that takes pointer