After Crystal 0.15, compiler will require declare the types used
by instance variables on classes.

This require changes to the usage of `Radix::Tree` by introducing
the type of payload elements it will handle:

    # Will only support symbols as payload
    tree = Radix::Tree(Symbol).new
    tree.add "/", :root

    # Error: cannot add node with anything other than Symbol
    tree.add "/meaning-of-life", 42

The changes ensure future compatibility with Crystal and also
enforces a more declarative usage of `Radix::Tree`.

If necessary, you can combine multiple types to ensure a tree
can contain all the wide range of payloads you need:

    tree = Radix::Tree.new(Foo | Bar | Symbol).new
    tree.add "/", :root
    tree.add "/foo", foo_instance

This change includes:

- Tree, Node and Result has been updated to require types.
- Node is capable of have optional payload (from defined type).
- Documentation has been updated to reflect this change.